The minimum time is 30 minutes, If you have used a bronzer you will want to wait 1-2 hours so the bronzer has the full time to develop on your skin. Otherwise you risk washing the bronzer off too soon and not getting the results that you wanted.
If you put accelerator on, you should wait about a few hours. When I go tanning, I take a shower that morning, go tan sometime during the day, and wait to shower the next morning.
Generally it is always best to shower before you tan. First of all this increases oxygenation of the skin and it takes UVA + UVB + oxygen to tan. Second, lotion absorbs better just after you shower....