How long does it take for Retin A to work?

So here's the deal: I had moderate acne, and OTC products wouldn't work. So I went to my doctor and she prescribed me minocycline (an antibiotic), clindamycin phosphate (a topical antibiotic), and Retin A (or tretinoin). And btw it's called Retin A Micro gel, 0.04%. I take the minocycline twice a day, use the clindamycin twice a day after cleansing, and the Retin A at night. My pimples are all gone (thank God!) but I'm left with a bunch of acne marks. I know they'll fade eventually, but they make it look like I still have acne. I know the Retin A will help fade them, but I'm wondering how long it will take (approximately)? I've been using this routine for three weeks now. Along with cleansing, I also exfoliate every other night.