I had an awesome overnight with Serena Rossi before she left for NY, was exactly what you always hoped for.
But I've also had not great experiences where the girl wants to be asleep by 10, wants a separate bed and sleeps in, then left with no morning fun at all.
My advice would be:
1) make sure you've seen her a number of times (3-6 at least, at least a few of them longer in duration) to make sure it's someone you'd like to see for 12 hours.
2) Be realistic in your assumptions (she will want to get at least some reasonable amount of sleep, she may have to call some folks at some point and need some privacy for that etc.).
3) Be clear on your expectations. Are you a cuddler? When does it start? When does it end? Some women think it means they show up in the late afternoon and leave sometime after brunch, some have a fixed number of hours in mind, start as early or late as you like, but 12 hours from now they are done. Are you a one and done? Are you someone who goes 6 or 7 times if you have enough hours to fit it in? Do you like playing in the morning - not everyone does.
4) Ask her if there's anything in particular she'd like to tell you about her overnight experience, you get surprising answers sometimes, everyone has their own take on what the experience should be