How long does a self-done spray tan last?

My skin color is very pale and I do tan in the summer but I'm still very pasty next to everyone else. I wanna buy a spray on tan or a rub on lotion tan, something at home. I don't wanna go to a tanning salon because I don't have the money for a membership and would rather not risk skin diseases. So once I spray tan myself, how long will the tan stay on for about?