The answer to this depends a little on what your goals are?  Are you treating acne scars, wrinkles, texture problems, or sun damage for example?  And most studies were a series of two to five treatments, depending on the study. 

I think 1-2 months in between makes the most sense.  You want to be healed completely from the first treatment before doing the second and make sure there are no problems. Also, it takes a month or so to really start seeing the effect of the new collagen production.  If you wait too long between treatments, you lose momentum and could easily get discouraged at the slow pace of improvement.  Expect to need a series unless your skin is fabulous and you are just doing a once yearly treatment for maintenance.  Here is a video demo of a Fraxel laser treatment.  Dr. I