Depends on what kind of tan you get. For a spray tan you should wait at least 8 hours after getting the spray tan to take a shower.
But if you're getting a tan in the sun or from a tanning bed then there's no need to wait any amount of time to take a shower.
If the tan is from a tanning bed or the sun you can take a shower at anytime. The only time you need to wait around 8 hours after getting a tan is if you get a spray tan.