How Long After Fraxel Repair Can I Start Using Retin-A

Obviously the physician who performed the laser resurfacing should be consulted first prior to resumption of topical Retin-A. You want to make sure that the wound healing is complete and that there is no issue with prolonged healing or redness for at least 4 weeks after Fractional CO2 laser before you resume topical Retin-A.

The dermatologist or plastic surgeon who performed your Re:pair procedure will be able to correctly evaluate your healing, consider your skin type and history, and best decide when you should restart the Retin A (which will indeed help the collagen stimulation further).
It definitely varies from patient to patient, and it also depends on the aggressiveness of the settings used. Be sure to use adequate sun protection for best results.