Tis one of the ways that my girl knows where I am at with my habit/addiction on any given day, actually...
If I'm high, I can last as long as she likes... she's not normally multi-orgasmic form penetration, so I'll usually get her off then slowly taper down into just cuddling for a bit then call it a night, because I KNOW that there is no way that I'm going to cum. Not when I'm well and truly high.
If I'm buzzed or coming down or whatever, I can usually go as long as I like, but not always. 45-60 minutes is probably the average, regardless if she/I orgasms (i tend to lose interest some/most times when i'm coming down and there is more dope to be done. sad but true) I don't always cum when buzzed, in fact less and less of late, but that probably has more to do with relationship issues than pharmacological ones...
not high/in withdrawals? don't fucking touch me is my usual stance, though my girlfriend will often ignore that wish and hop on anyway. in which case i last somewhere in the 1-3 minute range.
At first she liked the fact that I could go and go and go... now that she knows the source of my longevity in bed, she's not quite as enthralled and it is an issue between us every now and again.
If I'm completely sloppy high, I can have a bit of an issue getting hard, though its nothing that a few minutes of head can't remedy.
sometimes, I do have a problem with losing interest and going from rock hard to just sort of hard after a 20 minutes if i'm high, though.
in the end though, i'd choose a good opiate rush/high over sex with my girlfriend any day of the week. definitely not the healthiest relationship, considering she is head over heels in love with me. sometimes i feel bad about that, but, then i just go shoot some opiates and it doesn't seem to bother me as much...