This is a matter of how much you want to make per hour. How much do your services and experience worth to you? It also depends on the deal you have with the gym/studio you are working at. If you are an employee there I don’t think you have that much of a choice. And of course you have to stay competitive with other trainers in that gym or the surrounding area (unless that is not an issue for you). I don’t offer 30 minute workouts and for a 50 minute sessions I would charge the same rate as for a full 60 minute one.