I lost 100 pounds in 55 weeks, starting smaller than you are now. If you're consistent, the weight comes off easier at first and then slows. If your goal weight is considered "normal" for your height, then taking off that last 10 or so could take a while.
Assuming that 18 months from now you've lost a lot of weight and completely changed your life, I have a feeling you'll be thrilled, whether you've lost 100 pounds or 150...
So, like others, I'm cautioning you against setting fixed timetables where you will feel like a failure if you've "only" lost 100 pounds in that time...
I also want to say you can do this and not starve yourself to death. The goal would be to have a plan that's sustainable for weight loss and maintenance.