The answer to this really depends on your goals and your timeline.  If you have a wedding in 3 months it might change your planning.  In general though, 4-8 weeks is optimal. You'll see good results with a series of treatments and be limiting your risk of pigmentation problems.  

If you have a darker skin type, asian skin or a history of melasma, wait 8-12 weeks and absolutely avoid light exposure outdoors or triple sunscreen for a MONTH before and after each Fraxel treatment.  Be especially careful if you are on oral contraceptives.  I do not recommend this laser if you have active melasma - the most recent data shows it may cause it to worsen.   Dr. I