Roommate has already been evicted, but stuff is still here. How long do I have to give her to get her stuff out and if she doesn't get her stuff out, can I keep it?

If you were her landlord, you have to follow tenant law of your state, which you can find online. Landlords usually have to store possessions for X number of days and send a certified letter to whatever address they have on file, even if it comes back. If you are a tenant yourself, you need to notify the landlord that the stuff is there.
For all practical purposes, you could probably dispose of some of the random trash that roommates often leave behind when evicted and stack the rest in a corner. But be careful about that one person in a thousand who comes back claiming you stole her things.

DEAR chrishall, PLEASE don't leave negatives for opinions.
'Thanks anyway,' what does that mean?

First, please review the rules of this site on the use of the Ratings. I don't understand how you can say Joy's response was not helpful. If you have a follow-up question, then use the Answer options to post it.

As she noted if you were the roommates landlord then you need to give her 30 days notice to remove her belongings. If she doesn't, then you can dispose of them.