We use ClearBox radius server (on Windows) which uses a MicroSoft Access database.
The SQL statement we use is
Select Password FROM Users WHERE Name='$u' AND ValidTill>Now()
where $u is the username and ValidTill is a field in the database which stores the date/time when the user will run out of credit. The database is populated by some asp pages, on which the user signs up (with his credit card; he can buy a day, a week, or a month).
I suspect that all this is very ClearBox/Windows specific, and if you are using FreeRadius/Linux, this will not be very relevant. But anyway, I hope it helps.