So I totally misinterpreted the question. I put short onesies on underneath their clothes till they were walking around. But onesies as a single item of clothing, I stopped at 3ish months except for bedtime.
I categorised all one pieces are onesies, including the long sleeve footed ones!
Ok strictly as a piece of clothing with snaps at the crotch with no legs, I think I stopped with J after a few mths. As you can see though, I kept J in onesies with some sort of leg on it for much longer.