[skennedy]

The approach I have always been taught is to make it so the user does not hit the "pay wall" within the first hour. First full day of use is preferable. If they are using the product for that full hour, their use of the site/product is becoming more than a passing interest. In which case, when they get to that barrier they are already invested mentally and now they need to decide how valuable your service is.

In the case of Dropbox, you can store quite a few documents, pictures, and MP3s within 2GB. When you want to start backing up your whole hard drive, then you have committed to their product.