Hi Cathy,
I had mine professionally whitened about three to four years ago, and my dentist had two different techniques. The one I chose was the quick method, where you have the whitener spread on your teeth three different times and sit under a special light, (a laser? I don't remember), for twenty minute sessions. The whole procedure took two hours with prep and everything. That cost me five hundred.
The other procedure is where they fit you with trays that they make from molds of your teeth, and you apply the whitener yourself every night for two weeks, four hours at a time. That cost about two hundred and fifty, if I remember correctly. And like I said, that was a few years ago, so I don't if the cost has changed or the techniqes have improved. I will add that with the technique I chose, I also got the trays for my teeth so I could maintain it afterward. I never use them though, because I find that one tube of Colgate's Simply White Toothpaste, which takes me about two weeks to go through with regular use, does the trick every time.
I was very pleased with the results. I got a lot of compliments on the results back then, and I still get comments on the whiteness of my teeth even now. But I will say that if you are offered the two different methods, I don't see that one procedure is any better than the other. My girlfriend went the two week route, and her results were just as good as mine. I just liked the instant gratification that I got with the method I chose.
Good luck, and let us know how they turn out.
Jennifer