Massage therapy requires lots of physical exertion, so you must be in great condition. Your arms, hands, and fingers will get very strong and will require constant work to keep them that way. It is necessary to be able to bend, lift, stand, and maneuver all around a massage table and over a client. You'll be lifting legs and heads and arms, which requires a level of fitness many people don't have.
Considering how physically demanding giving massages can be, most practitioners limit their hours to about half of what a normal workweek would be at a conventional job. So, take that into consideration when estimating how much you can make. If you work for someone else, you'll get an hourly rate plus tips -- a fraction of the price the customer's paying for the massage. If you own your own business, you get more of the money, but you also have a lot more costs -- rent or mortgage, utilities, insurance, furniture, consumable supplies, repairs, accounting, and staff.
Consider also your level of squeamishness. If you get a client on the table who's really hairy, or has skin lesions, or is generally unattractive, would that gross you out? Would you be able to work with special requests even if you don't understand them? Are you a good listener? How would you handle a client telling you that you're doing something wrong? Are you comfortable dealing with clients? Some people prefer to sit behind a desk and not have that many interactions.
The years of school required depend on what level you want to be at and how many kinds of massage you want to learn how to do. Look into your local massage school's requirements and arrange to interview current and graduated students. If possible, visit working massage therapists at their practices and ask them about their work.
Keep in mind that any career that involves physical labor carries the risk that if you injure yourself, you need to have another source of income to back up the times you can't work. One massage therapist I had developed arthritis in her fingers as a young woman, rendering her unable to work as many hours as she needed to survive. Don't underestimate the stress your body can be under when doing massages full-time!