grad school dependent

Our dependent 22 yr old grad school student works 2 part time jobs. we have been claiming her and the school deductions right along.

How long can we claim her as long as she is living at home

and How do we know if her return would be better if we didn't claim her? As of right now she's still low income under $20,000. by us claiming her is that reducing anything she would be entitled to other than stimulus?