For an undergrad, how long should a short-term volunteer position in a lab be?
I would refer to the way university offers undergrad research courses.

For example, at my university, students can work in lab in summer (12-16 weeks, full time, 40-45hrs/week).

Often, assuming both parties like the work, the student continues as a research course student for the next school term. In this arrangement, nothing changes except the compensation - in summer the work is paid, during the school year it is for credit.

Now, from what I have done and seen, people don't stick in the same lab longer than that ESPECIALLY if you are looking at grad school or medical school. This is because you need 2-3 PIs giving you a glowing reference.

That been said, I have taken a summer research project which I didn't continue (I.e. the length was 12 weeks). And I've done one research project which is 2 years (and still on-going). My third project was summer + school term in length.



In short, you should diversify your research experience in different labs. You can work as long as 12 months, and as short as 3 months in the same lab.

Though I don't know much about other universities, I expect this to be same or very similar at other universities. The time to learn basic skills will vary but 3 months should be a good start, of course as you do something longer you will get even better. But doing work in different setting will definitely help person get better with basic skills because different labs operate differently.