It's different for everyone. It was so easy for me that I didn't even have to study. I passed the national exam the first time. Others in my class struggled with it and failed the national exam at least once.
Most of your studies will be on anatomy and physiology, and learning the muscles. You will learn every single muscle, its origin, insertion, innervation, synergists, antagonists, and action. That can be pretty tough for some people.
But massage school is not like regular college where you only have one shot at passing or failing. The teachers will walk you through it as many times as you need until you get it right.