how many hours does a typical overnight session consist of?  

thanks

You need to ask the provider.  I have had overnights as short as 5pm-8am to longer than 24 hours.  I remember one who quoted 11pm to after breakfast which sounded like she had something to do in the evening.  I passed on that one.

I would note if they travel to you, at least in the back of your head you need to consider travel time.  I once had a provider spend 10 hrs trying to get to me because of weather.  I did appreciate her dedication in making sure she showed rather than giving up and going home.

Simple answer --it varies -so talk with the provider you want to see. Most of the ones I know have varying rates for varying numbers of hours. No 2 are the same. Just have to ask. Make no assumptions and if you want to wake up in the morning with someone, make that clear too.