In the holidays it doesn't matter so much does it? Unless it's happening every other night or unless lack of sleep makes them evil. DS1 didn't manage any sleep at his last sleepover party which was in June , because they were camping in the garden and it was so light.
I wouldn't expect much sleep, or any at all on the first sleepover. Later ones, they drift off at about 2-3am. I don't think there's any problem with that unless some of the sleepover-ites have an early departure the next day or something special they are due to go on to. Mostly, so long as they are quiet, I don't see any issue with them staying up as long as they like.
If they are trotting about the house, squealing or nabbing food from the kitchen, after about 11.30pm, then they get a major bawling-out. The house has to be quiet from then on, but they can watch films on the laptop for as long as they like.
We only have sleepovers when nothing important is happening the next day, and we have one basic rule at sleepovers - you can stay awake as long as you like, but if I have to come downstairs to tell you to keep the noise down more than once, you'll wish I hadn't. Works very well, on the whole. They mostly go to sleep about 2 am, I'm told.
When my 14 YO has sleepovers I try to encourage her PC off at 10pm but don't enforce it. (Usually they have to have it off at 9-10pm depending on school night etc) I am stricter with my 12 YO on this. Usually they just play on the Wii in their room, listen to music or watch DVD's and I get them in some popcorn and a bottle of coke and a pizza. They are usually happy with that and I am very grateful as i was up to allsorts at that age!
Forgot to say that usually they go to sleep between 12-3am. Usually still up 8-9ish though. They are usually not that loud (they are not directly next to our room) and I put a fan on to drown the giggling out.