Good question but so hard to answer. There are too many factors -- age, amount of cycles, years training prior to juicing, time off in-between cycles, commitment to training while off cycle, etc. So many guys don't reach their natural potential before using roids so how much did the roids help? Impossible to say. Even if the person answers honestly, in their mid, it may not be accurate.
I think it's possible to add maybe 10-15 pounds of actual muscle tissue from gear over the course of a couple of years and keep them. Anything over that is either maintain by continued cycles or was mass that would have been obtained naturally or it's fat.