This is partially your preference!
The most important thing is that the old spray tan is completely removed before getting a fresh one!
This is why it is essential if you are an avid spray tanner to have a good exfoliant mitt and spray tan friendly scrub! A tan remover mousse will also be your best friend when you want to keep up with spray tanning.
A good rule of thumb is waiting at least 7 days between sprays considering you are exfoliating at least couple days before the next one.