A question like this is rather like asking "how long is a piece of string?" In summary, most teachers here try not to do more than 16-20 "contact" hours per week with their principal employer.
However, many schools in Thailand will not allow you to only turn up for your 'contact hour' teaching. There is a very genuine expectation that you'll hang around the school doing your preparation work, marking, etc. This is also for the school to advertise the fact that it does, indeed, have foreign "native-speaking" teachers.