The better you care for your spray tan, the longer it will last. Make sure to listen to the time frame your therapist has told you to leave the spray tan on for. This might be 2-4 hours or could be up to 8 for overnight. With some tans the longer you leave it, the darker you will be but be sure to follow what your therapist has advised depending on the tan they have used.
Try not to get caught in the rain following your appointment as raindrops may distort where the tan has been applied and you may end up with small white dots where the rain has landed. Likewise, avoid washing your hands or feet following your treatment until the 2 hour mark has been reached. Try not to get too hot and sweaty immediately afterward (no gym sessions or running until after you have showered!). Until you shower, wear loose baggy clothes to ensure the tan can develop freely.