Prepaid billing using freeradius and rlm_counter/rlm_sqlcounter

Hey guys, I'd like to try and implement a solution for pre-paid accounts. The idea is that I'd be providing pairs of username/passwords to customers and these accounts will be set to N hours, after those N hours are used by the customer (the user has ran out of his N-hour time) the account will be disabled, i.e: freeradius would return an access-reject. I'm currently using FreeRADIUS 1.1.0 with mysql as the back-end on debian and I've read the manpage about rlm_counter and rlm_sqlcounter though the manpage is a bit low of information and examples. Could someone please provide me with an example of how to get freeradius configured to this solution? Thanks, Sincerly, Liran Tal.