It greatly depends on your professor that you work with.
My experience is that you can always decline it if you see that there is no fit or if the time commitment is too high.
Usually these assume a certain number of hours (such as 10 or 20 hours per week). Sometimes it ends up being just an hour a week or lasts only half the time since professors don't need help the entire term. In other cases, it may be something that you spend quite a bit of time on.
In 90% of the time it is very much worth it:
- Easy money
- Usually interesting assignments
- You get to know the professor (which depends on the prof - may be an opportunity of a life time esp. for some stellar guys)
- You can decline/quit though it will be a sour point in the relationship with the professor
Generally this is very positive and students compete for it.
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