no more than two days . I had one a couple of months ago. Had it done on Thursday , do on Saturday . Becareful how much you shower etc if you wnat it to last for your event.
Make sure you exfoliate and moistirise in the run up to the spray but dont moisturise the day you have it.
If it is a day do I wouldn't opt for a spray tan I would go for a St Tropez etc as I found that the coverage wasn't brillant for the bits I needed done i.e back.