My longest session has been 3 hours. I don't grok the allure that overnight sessions apparently have for some subs (perhaps because I am more masochist than sub. Three hours of mostly pain is a lot of pain). One thing I've noticed while perusing websites is that dommes who have the same rates for hourly sessions will often charge very different fees for overnights. And some don't do overnights.

I went through a phase where I had a strong opinion that double domme sessions were not worth the money (and can be train wrecks if they don't get along). I have revised that opinion in recent years and do a double occasionally.