Attending a spin class once a week or even once every other week will provide a decent workout. Spinning is a great source of cardiovascular activity, and most people can burn 500 calories or more during one session. Depending on how much recovery time you need and how quickly you want to get fit, you may wish to go more than once per week.
Spinning promotes an active lifestyle, but you shouldn't overdo it. By hitting the spinning room too many times in a week, you can do damage to various parts of your body, particularly the knees and hips. There is even the chance that you could develop tendinitis.
You would be best served by attending two to three classes per week. That level of activity will improve stamina and increase weight loss and toning while reducing the chance of injury. The fact that it is also relatively easy on your joints means you should have no trouble doing it a couple of times a week.