Well. Twice used to be the average for me. Three time if I really fancied the girl. The most I managed in an hour was 4, a couple of times.
Recently, once seemed enough. Until I realised that it wasn't my libido, it was boredom. Same thing each punt and I noticed the mechanics of it more.
My advice would be to go for fantasies, or girls who you are really really attracted to, get that thrill back that you used to get. I went back to see a girl from a few years ago, that was just my type, and the libido was fully back. I'd not worry about drugs yet, just spend a bit more time planning and picking the girl than you would if you were 20 and would hump a lamppost.