How long after stopping Retin-A can I tan?

If you must tan, which we would strongly suggest against, then 7 days off Retin A should be sufficient. Retin A thins the stratum corneum, which is the surface dead skin cell layer. Stopping Retin A causes a buildup of dead skin cells on the surface which offers very little but some additional protection from UV rays.
Don't tan, but if you do, go off your Retin A at least 7 days before.

Never. No dermatologist can ethically tell you that you can tan!!! Use a sunless tanning spray if you need the color but you are exposing yourself to radiation (ultraviolet) damage which will make you look much older eventually and increase your risk of skin cancer considerably! Sorry.