There are typically three things evaluated for tenure: Research, Teaching, and Service. The importance of each depends on the institution, but at an R1 you can guess that Research as evidenced by quality publication is by far the most important. At small colleges it would likely be Teaching first, but not necessarily. But no candidate can afford to not have some evidence in each of the three areas. For some, a book (or three) might be evidence of Service, or of Teaching. Only a seminal book would probably count much in the research area.
In many ways the quality of a publication record is best measured by how much the individual has influenced other researchers in the field in question. How often have they been cited, for example, or how much work has been done to extend the work of this candidate. In other words, how central is this person to the current work of the field. For a mid-career academic, writing a lot of papers may be necessary, but not sufficient.
The kind of person who the institution really wants to tenure is the kind of person who they really don't want to lose. For an R1, think of the following in addition to the publication record. Does the candidate properly mentor grad students? Does he/she hold advanced seminars or run labs? Does he/she bring in exceptional visitors to lecture or talk? What have the advisees of this person gone on to accomplish? Does the person mentor junior faculty effectively?
In some fields, such as CS, in which conferences are important, is the candidate regularly represented? Does he/she participate in conference committees? That can be evidence of service, but also of having a central place in the research community.
At some institutions, grant funding outweighs a lot of other things. One wants to say "outweighs everything" but that seems a bit cynical. But if a prof can draw enough grant money to fund grad students it will be hard for the institution to let them go.
Another factor, not usually formally weighed in, is how collegial is the person. Is the other faculty happy to share the coffee room with them? Note that it is the more senior members of the faculty who are going to be making the decision at most places.
At many universities the candidate creates a dossier for examination by the tenure committee. The dossier may be used to a greater or lesser extent to guide the discussions. But the candidate gets to make their personal "case" for tenure. If you have an idea what they want to communicate and accomplish in the dossier, it might be helpful to both support that but also to supplement it. Sometimes it is hard for a person to say good things about themselves and others can do so more easily. I once mentored a candidate who couldn't bring himself to point out how great he was.
Some places will even formalize the mentoring process for a candidate. Ideally this starts a couple of years before the tenure decision when the person still has a chance to enhance their record in appropriate ways. I've mentored other people (for promotion, not tenure) who refused all advice and didn't advance.
The time to do such a recommendation can vary widely depending on how well you know the candidate and his/her position in the field. If you have a lot of personal knowledge and work in the same field it might go quickly, but I doubt that a day is sufficient. I would want a week, actually, but might do better in some situations. I put an important recommendation together (for an award, not tenure) in a few days. Part of that was just thinking about presentation, so wasn't full time work, but the ideas needed a bit of time to settle and mature.
It can also vary depending on just how important your recommendation is. If it is expected that your support is vital, then it naturally takes more work if you want to be helpful. But if you are just a minor commentator and others will bear the major weight, then it can be quicker.
But, I wouldn't do it unless I actually wanted to do it. Just repeating things you can easily find (Google) will be less than helpful. If you have no personal knowledge, you aren't the right person.