What length of time can i have a visitor stay with me for by law, in my apartment.

The issue is not the law but your lease. It should specify. No lease, no limit. If this is about public housing, there are strict requirements.

I agree with attorney Tumposky. If you look at your lease, there may be a clause (sentence or paragraph) that specifically speaks to overnight guests. If there is no clause or no lease in the first place, there is no standard at law. As stated above, there are strict rules in public housing, including government-assisted housing (Section 8) and it should be in your paperwork.