I've been a part of lots of erotic hypnosis sessions, mostly as a 'tist (or hypnodomme, if you prefer) but also as a subject.
Live sessions are infinitely better than recordings. It has to do with adjusting to feedback, establishing rapport and knowing your subject (or your 'tist knowing you).
Ideal session lengths vary greatly, but in general I like at least half an hour, with time for aftercare if it's a more formal session, with the longer (and more interesting) the better up to a point of exhaustion on all participants parts. For conversational trances, especially subjects I've worked with before, time isn't so much of a concern, it's not hard to get into their heads and move some things around 😉