Other than for monetary limitations, I think that the length of the session is to some extent dependent on the activities planned for the session. Like most others here, I also started with 1 hour sessions that allowed me to explore and experience many different types of play.

I quickly learned that I had a very strong love of heavy leather bondage along with heavy NT, electrical, anal and CBT play. The rigging for these scenes is very complicated and time consuming. They often involve multiple changes of restraints and equipment that require more than 1 hour. I have been doing sessions of at least 2 and usually 3 or 4 hours for some time now and that seems to work out very well. I also do an occasional heavy bondage T&D session and those are great fun at 2 hours.