Clenbuterol is a very harsh stimulant and puts a lot of strain on your heart. I personally wouldn't run it longer then 10 weeks.
I personally am not a big fan of clenbuterol.
Clen simply raises metabolism. It does not differentiate between muscle or fat. Clen might raise your metabolism an additional 100-200 calories per day. You could just easily do an extra 15 minutes of cardio. I have used clen in the past and I always felt it did more harm then good. It caused muscle cramps when I was training and it really negatively impacted my sleep.
There are better alternatives that directly increase fat loss, with no risk of loosing muscle mass and no side effects like insomnia or muscle cramps. Cardarine gw is going to be your best option. It directly increases fat loss, endurance and recovery. It is not a stimulant or a hormone and is pretty much void of side effects.
I haven't used clen myself in years and don't recommend it to my clients at all anymore.