I've been a few times. I think a good session is 90 minutes. 60 seems too short for me but 90 is good enough to get the full experience. You really have to know how to relax though.
There's one in San Diego called Flot and the owner recommends at least two hours. I booked it and must've floated for closer to three, which was about right. It had three stages:
1. Mind racing, everyday worries at hand (~45-60 minutes)
2. Thinking through actual concerns and whats important (~30 mins)
3. Complete relaxation
It's an incredible experience if you devote the time to it, but to really feel it you need to get through the first stage which can take over an hour. I left happier and more peaceful than I had been in some time.