Blood work will always tell you but as a general rule, time on + PCT should be your time off. So a 12 week cycle with a 6 week PCT should be 18 weeks off cycle.
Blood work will always tell you if it has been long enough but if you want to be safe, follow the formula of time on + PCT = time off. So a 12 week cycle and 6 week PCT means you should be off at least 18 weeks between cycles.