I've broken 7 different vertebrae on two separate occasions. The second time I also had a herniated disc and wore a back brace for 6 weeks to heal. I still have back pain, but usually just when I'm straining it too much just like it would before I broke my back. After I took the brace off, I was riding coasters and other rides within a matter of months. Every situation is different though so it would be best to ask a doctor first. If anything, try tamer rides first and work your way up to wherever you are comfortable. I'm sure it depends on a lot of things like your age, physical form, what your body is used to and how tight (or how loose) a certain ride's restraints are on your upper body.