Since different professors are different people, there is no golden rule on when exactly professors reply to mails, but the following guiding principle may do:

**E-Mail by students should always be written in a way that the workload to answer them is not unnecessarily big.**

There are many ways in which this can happen for mails by students:

 1. No proper sender name indicating how to address the student, which makes it difficult to reply with the correct "Dear Mr. XY" or "Dear Mrs. XY".
2. Asking for information that is clearly given in the syllabus, is written on a slide, or has been clearly stated in the lectures
3. Not making a proper effort to describe the problem clearly and concisely. For instance, after advising a student on how to solve a specific problem with the thesis research, the e-mail response by the student should not be "I tried the approach you suggested but it does not work", because there are four problems with this answer:
    - It ignores that three approaches were suggested.
    - It does not state what the approach was that was tried out (a professor cannot always remember, especially if the e-mail comes after a week or more)
    - It does not state what the student expected to happen
    - It does not state *why* something did not work, what the observations are, etc. This means that the first thing that the professor will need to do is to ask for details, which is a waste of time.
3. Asking for unreasonable accommodations
4. Having grammar that is ambiguous enough to require guessing what the student may mean. Bonus points if the e-mail is vague enough so that the student hopes that the professor will just write the solution even if the student cannot articulate the question.
5. Not using the official university e-mail address, so that it is unclear if the e-mail really came from a genuine student.
6. Not mentioning the slide number when asking a question about a specific point on a slide.
7. Asking more information on a topic when there are already references and explanations given in the documents for the students, without mentioning why the may be insufficient.
8. Including sentences that appear to be attempts of emotional manipulation by the student.

There are likely to be many more examples on how an e-mail can add unnecessarily to the workload of the professor. E-Mails adhering to the principle of not causing unnecessary work are typically also formatted correctly. At the same time requiring proper formatting also means that the student has to put thought into an e-mail, which incentivizes requiring students to format their e-mails.