Come on, Wayne Rooney still looks good and I think the hair he's losing are the remaining hairs in the male pattern baldness region.
The hair at the back and on the sides is not affected by DHT. That's why it's called male pattern baldness, if all the hairs were affected, then it's just alopecia areata/universalis. So yes they'll last forever.
If you're on anti-androgens, those hairs will last until you start to get serious side-effects or want to have children, or propecia is taken off the market because of safety problems. So yeah maybe a long time since a some people here don't want to have children and the pharmaceuticals are greedy.