Yes, they should come in different shades to suit different skin tones, and different levels of base tan. They normally suggest you start with a lighter one the first time, and then go deeper on subsequent occassions.
In terms of how long they last, it depends on the condition of your skin. If you do hair removal and exfoliation and thorough moisturising 2 days before, then have a hot shower before you go to open your pores, it could last 7 days quite well.
Don't go for a St Tropez. I had one done and it was rubbish. Just not dark enough. Barely a colour change.
The beauty salon I go to use two different types, one's called california tan and I can't remember the name of the other one. I think it's the california tan whereby your beautician can as you how dark you want to go and select a colour appropriate. I think it's done by a percentage - most people go for between 40 and 60%.
I'll ask them for more details if you want.
You need to exfoliateyour skin before you go for a spray tan so that it lasts longer. With this preparation it should last 5 to 7 days. Though you should remeber to only pat dry your skin after each shower and use lots of moisturiser (something like cocoa butter is good)