How long does a petition take for a child over 21 and the parent is a us citizen

Depending which country they are a citizen of.. it can be a very long time.

It depends on whether the child is married or unmarried. Every month, they put out a Visa Bulletin (http://www.travel.state.gov/visa/bulletin/bulletin_1360.html) that indicates which priority dates are now current. According to next month's visa bulletin, if unmarried, they would be in the F1 category, which is now current for priority dates from 7 years ago (except people born in Mexico and Philippines, which are much longer back); if married, they would be in the F3 category, which is now current for priority dates from 10.5 years ago (again, except people born in Mexico and Philippines, which are much longer back).