How long after stopping Retin-A can I tan?

Tanning is only damaging your skin, You should always use an SPF when planning on having any sun exposure.  I would NOT recommend tanning!!!

If you are using Retin A properly you should have been instructed not to be exposed to excess sun.  The use of Retin A is typically done over a period of time, this results in thinning of the protective keratin layer as well as reduction of the pigment cells.  Both these facts will leave you more vulnerable to the UV radiation.  Not only you should not tan, but you should protect your skin with a complete sunblock to avoid damaging your skin permanently.  

If you stop using Retin A and your skin regains its normal color and texture then you can tolerate the tanning better.  However, this is not a good thing to do for your skin in the longrun.  

Irregardless of your Retin-A use, do yourself a favor, stop doing tanning bed! I was quoted by my published article, along with scores of other peer-reviewed articles that have demonstrated a link between skin cancers including melanoma and usage of tanning bed. See my quotes from February 9 2010 issue of BusinessWeek (http://www.businessweek.com/lifestyle/content/healthday/635508.html). With usage of Retin-A, sun sensitivity is really not an issue as long as you wear sunscreen daily as you should anyways.

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.