I like the half hour sessions for the "dirty-afternoon-quicky-with
-your-mistress" feeling of it. I prefer them with clients I've already met before.
For new clients, my preference is for a one hour session. Since I don't screen and book my own appointments, I prefer them to multi-hours dates: two hours can last forever with someone you have nothing in common and no chemistry with.
My ideal date is 2-3 hours with a repeat client: we already know that we have some chemistry going, there's enough time to not have to worry about time, it's more relaxed, it feels more like a "real" date with a BF or fuck buddy, more time to try out different stuff, etc.
For duos or couples, two hours is the minimum, three hours is better. There's always a little bit more chit-chat when more than 2 people are in the same room, more giggling, more breaks in between orgasms and positions, etc. There's also so much more to do and try out with more naked bodies in the room.