It wouldn't be a hell, considering this:
1. The women would go insane only after week or two after entering the vault. The guy has plenty of time to prepare himself.
2. I doubt that all the 999 women would wake up at 8 o'clock, rush to the man and gang-[censored] him. There would be turns, and the man would have to do it in about 4-5 times a day.
3. Considering that a normal person can live normally some 4-7 days without six, the turn system would be good.
And there is always chance for lisbian relationships, too.
Though it has to be admitted that after 12 years of this the thing would feel kinda boring.
If the people are ~20 years old when entering, and die at 80, it would be 60 years of this? I doubt.
Also, if there is only one man there, the children couldn't reproduce. Because everyone has the same father, the vault would die out in two generations.
1. A week? I'd expect mass hysteria on the first or the second day if the average age of the inhabitants is about 20. Think in terms of "Oh my god!!! The fluorescent light just blinked! The vault is failing! We're all gonna die!!! Quick! Let's find the reactor thingy and FIX IT!!!".
The only thing he could plausably do to ensure his (and their) survival is to find a place to hide and wait it out the first few weeks, preferably undetected at all (if possible). And later claim hitting his head and blacking out or something. A good strategy would be to do it by barricading himself around the
other crucial systems - reactor, water/air recycling, food growing etc. and protecting them from destruction by panic. Preferably by locking and blocking a door or two.
2/3 If he makes 4-5 happy per day, it would take him 200 days to get them all done. I'd expect that by the time of N999's turn, she'd be like the gals from Futurama's Amazon Women in the Mood episode
His only hope is to reduce the waiting list for the second turnaround by impregnating as much of them as he can, slowly spreading the load.
4. With a bit of careful planning and a lot of luck he might make the gene pool a bit less shallow. It's just that the sons of his would have to "help him out" with the remaining waiting list rather than working on their peers. That way, depending on ages, with careful log keeping... Maybe the gene pool can get just deep enough? (Of course, if we're talking average age 40, no chance, but with 20 maybe even the grandsons can help?)
DISCLAIMER: Written under temporal insanity and jet experimenting. Reader's sense of humor advised. Otherwise, kindly disregard