I'm not sure if there is a way to define how much data is used, but depending on your carrier you can log into your account and check your usage for the billing cycle and such.
Posted from my CrackBerry at wapforums.crackberry.com
Technically speaking, there is a total data as well as voice usage counter on all BlackBerrys, however unless the BlackBerry in question has been connected to a BlackBerry Enterprise Server, the counter will stop at 250K. This is due to an old warranty policy of RIM to see if a customer qualifies for exchange of device if it is still under warranty and the conditions are met (less than 250K data usage, less than 1 hour voice usage).
Try it out yourself and see what you get:
1. Go to Options > Status.
2. Type "BUYR" (you won't see any typing)