I'm currently finishing the last year of my undergrad degree, and plan on applying to grad school in about 7 years. I'm taking a year off to work, then completing a 1 year graduate certificate followed by a 2 year diploma program and 3 more years of work experience before applying for a masters program, which requires 2 reference letters from relevant profs. I know I can ask instructors from the grad certificate and diploma programs instead of relying on professors from my undergrad courses, but that's still a 3 year gap between my last course and application. Do I need to wait until I apply for my masters to ask for references? Or can I ask in advance before graduating my grad/diploma programs, after explaining my plan to apply for a masters in 3 years? My concern is my profs might not remember me...
No, you do not need to wait, you can certainly ask in advance. There is no rule against it. However, if the readers of the recommendation letter notice that it is from several years ago, then they may react negatively. On the other hand, they might not. Admission committees typically process many applications, so do not read any particular letter that carefully. It is unlikely they will check the date.
The concern that professors will not remember you when you ask for a letter years from now is very reasonable.
After doing other things for several years, you may ask people who know about those things to write letters for you. This would give a more up-to-date picture about your qualifications. However, you can both ask for letters now and after completing the years of other activity.
It is quite normal for students to ask their undergraduate professors for letters of recommendation, years after leaving the institution; this is especially the case for students like you who plan to work prior to graduate school. Many programs encourage some time off after undergraduate education because of the possibility of "burning out" (amongst other reasons), and thus many professors are used to the process of writing a letter for their student, years after the student has left the institution. Though 7 years might be a rather large gap in time (and some of these professors might retire or change institutions by then), what is important is that they are made aware as early on as possible that you will eventually want this letter from them.
Prior to leaving your institution/graduating, establish extensive rapport with those you are interested in receiving a letter of recommendation from and inform them of your future graduate school interests. Gauge the extent to which they are on board with writing you a letter, and ask if you can meet with them to discuss this letter at a later date, after you have been able to assess some sort of scope regarding what your application for graduate school will look like. Though this depends on the discipline, many professors do not want to merely be asked for a letter, but wish for you to present them with an early form of your statement of purpose and/or research interests first prior to asking for a letter.
After a professor agrees to write your letter, maintain email communication with them with updates, when relevant. There is no need for an inundation of emails with every little update, but it is helpful to check in with the professors. Keep a copy of your transcript on hand; this will be useful come recommendation time, because you are reminding the professor of the classes you took (with them and in relevant courses that they can speak on).
Though beginning the process for a letter of recommendation is subjective and you could ask the graduate coordinator of program(s) you plan to apply to about their personal stance on getting letters x amount of years ahead of time, to be safe, continue maintaining that contact with the knowledge of what each of your 2 letters of recommendation are asking for. Does professor X want a copy of your statement of purpose and your CV? Does professor Y instead only care about any updates after your certificate program? Each might have their own way that they like to go about the letter process.
Finally, when application time arrives, I would drill in the letter dates. Visiting the professors in years closer to the application cycle might be helpful, for meeting in person allows them to put a face to the name and CV; this part is completely up to you but does not hurt.
In sum, there are many ways to approach this, but approach those important professors early on while you are still fresh in their minds, check in with them as appropriate, and (if applicable) inform them of professional/academic updates that could affect their letter to you. 7 years is enough of a gap that your academic, research, and/or work goals may change in a manner that has you shift interests or consider changing who you want as your letter. Many changes are very possible of happening, so I would not stress about it too much.. just ensure that if someone agrees to write your letter, that they know that it comes with the asterisk of being a few years down the line, and hold them to it. There may be times that professors are far too busy with their own work to answer your check-in emails, but that does not mean you should not be adamant in keeping somewhat of a connection with them over the years through which you are no longer their student. The last thing you want is to have great people offer to be your recommenders, only for them to forget (since 7 years is a while away); of course, as mentioned earlier, many professors are used to writing letters years after their student has left an institution, but that continued communication will likely provide a stronger letter in the end, since you have been diligent in keeping yourself somewhat fresh in their mind.
Best of luck!
I think if you're seven years out, the reference letters won't be as relevant. I would check with the master's program, but sometimes if you've worked for a certain amount of time you can provide one academic reference and one work related reference. I would try to keep a relationship with at least one professor - keep them updated, visit for coffee, etc. but wouldn't use a letter from seven years ago to apply for a Masters program. Maybe through your diplomas which will be more recent, you will find a more recent reference.
Your concerns about not being fully remembered are quite valid. If you know (or even strongly suspect) that you will be requesting a letter of recommendation in several years, then I would encourage you to make the request now rather than wait several years as others have suggested. Talk to the professor and let them know what you are planning and explain the timeline. If she is willing to write you a letter of recommendation, then she will likely appreciate the opportunity to collect her thoughts while you are still fresh in her memory. If you wait until 7 years out, then I would expect that the chances of them saying "no" or writing a weaker letter would be much greater.
I was just recently asked to write a letter of recommendation for a Masters program for a student that graduated 10 years ago. They asked me because the program they applied to specifically requested a letter (or letters) from an academic setting and the former student has been working in industry in the meantime. While I distinctly remember the student, I couldn't tell you what classes we had together, what their grades were, or what departmental activities they were most active in. Had I known this request was coming, I could have either written a letter back then or at least jotted down some key facts that I would use in such a letter in the future.