Well, I've heard that you should see some kind of difference within 14 days for most products. But if I don't see any results, I usually give up after a week. I'm just not patient with these things I guess.
What I've done to get around this though is to buy starter kits when possible. Then they usually give you enough of the products to give you some results, and then they recommend you to keep using the products for the results to get better. So I figure that's a great way to try a new product or line out.