18 months is insane. MCAT studying is suppose to be a mini-marathon, not a lifetime. As @itsthat1guy said, 4 months will be adequate. 2.5-3 if you have nothing else on your plate, and maybe 5 if you'll be busy.
Most students study 3-4 months but some study much longer or much less, all depending on their knowledge and readiness. If you are taking classes then that is what you should focus on now.