7-10 days. Longer if you keep your skin moist and your exfoliating to a minimum.
Yes, you will want to make sure you have exfoliated your skin well, shaved and left off any deodorant, lotions and makeup. Also, wear cotton undergarments that you don't mind being stained.
No, the solution we use works with your natural skin tone so you won't look too dark or orange.
Even if you already have a tan you can be spray tanned to look darker and because of how the solution works with your current skin tone your results will be darker than whatever your current skin tone is at the time of the session. If you were recently sprayed and want to look darker you can by being sprayed again. The more layers of solution sprayed on the darker the results will be.
Yes, you will want to make sure you do not get wet for 4-6 hours after your tan.