No one can really tell you exactly how long it will take because there a lot of factors involved, most important of which is your dedication to getting treatments on time. 12-18 months is the correct estimate if you schedule your treatments in a way where you get full clearance as fast as possible so that you’re only coming in afterwards to clear the new hair that showed up as soon as it does.
Basically, it’s in your best interest to go in as often and for longer treatments up front to get to a clearance. And it’s in your best interest to come in as soon as you see new weak hair pop up. If you do this consistently, you’ll be done in under 18 months. If you skip here and there or take a while to get to a clearance, it stretches the time frame.