12 or 24 hours, although many ladies would be happy to accompany any timeframe you suggest. Many ladies also require 6-7 hours of sleep with an overnight.

This varies with every provider. Your best bet is to find a gal you are interested in and ask her. Not all ladies will do overnighters with a first time client and, quite frankly, I wouldn't recommend it for a newbie and especially not with a provider that you have not seen before.
Overnighters are not a non-stop boinkathon. There is time to kill and time to fill with the lady. If you don't have some chemistry it's going to be a long and probably disappointing night.

My definition...

My first overnighter I didn't know any better and we had sex like 5 times.  I got about 4 hrs sleep - and I was tied up for about 1/2 of that.  (meaning with rope...)

But in a testement to male prowness the gent was 73 years old!  And still one of my ATF's.  Giggle.

But the reality, now that I know better is usually an overnighter takes two avenues.

Either arrive later 10-11pm play around awhile, sleep, maybe a quickie, sleep, and set the morningwood on fire. Maybe breakfast, or I'll just scuttle out the door in the morining.

The other way, which I think is more fun for all.
A full evening dinner, show (whatever) date, come back somewhere between 11 and 1am and follow the above...

Personally I prefer a dinner date, as usually if it's someone I just met I'd rather just enjoy a romp and sleep in my own bed with my lover - but it's all relative, and money talks... and there's been a couple of rare occasions where I stayed all night for fun.

But don't count on it...

Giggle,
TS Jamie

Depending on when you begin, but normally starting at around 7pm, perhaps dinner, etc going until 9am or so, (so about 12 hours is the norm),unless you want 'breakfast in bed' *wink*