"Comment est-ce qu'il dure ?"
"How long" is a question of quantity in the sentence "How long does it take?" - quantity of time.
The "how" in the English translation (and to my understanding the "comment" in the French) means "by what means", "through what method". I could give salacious examples for the context of "How does he last?" but instead I'll suggest the following:
- How does he last the whole day without eating lunch?
- How does he last all week without seeing his wife?
- How does he last a marathon? He never seems to train!
the way dure is spoken is quite cut off, thus singular, as opp to doorrrr (with a prolonged guttural french style R) which indicates plural, so to my ears it's certainly singular.
I would say that I would say IT rather than HE in normal circs, unless its some sort of query about sexual stamina!