It can all depend on the clinical trial itself, but generally speaking if the treatment you are getting works and is stopping your cancer grow you can stop on the trial.

But if the treatment you are getting is causing you to be unwell, and you are not getting any benefit the clinical trial would stop then. Taking part in a clinical trial is completely voluntary, so you can decide to stop taking part in one, at any time for any reason. If you are asked to take part in one, this will all be explained to you before you consent to taking part.