You should try to incorporate FLs as soon as possible, once your done content review. If you can get as many FLs as possible, that would be optimal; however, you should take into consideration your studying capabilities. The last thing you wanna do is burnout nearing the MCAT!!!
With that being said, I find that you wanna mix up the FLs somewhat, so you do not accommodate to the question styles of prep companies but instead accommodate to writing an MCAT-formatted exam. It is key to finish with AAMC FLs though, as you want to be comfortable with the actual MCAT formatting (duh!). If you are doing just BR and AAMC FLs...This is my suggestion for the order of FLs:
-BR 1-4
-AAMC 3-7
-BR 5-7
-AAMC 8-11
Would be best to have a few days in between them, and don't forget to review EVERYTHING after you write each one! The basic principle, that I think would work best, is to "block" the FLs, as above.