weekly allotment?
There are some scripts that could do this
I believe ioA
weekly <user/=group> <section>,<MB>
Sets X MB credits for a user to be used. It does not add but sets it to specific value.
Example:
site weekly ioFTPD 0,500 Set ioFTPD 500MB credits
site weekly =ioftpd 0,500 Set ioftpd 500MB credits
site weekly ioFTPD 0,0 Removes allotment for ioFTPD in section 0
site weekly Lists users with allotment
I can't remember if monthly is in it aswell but maybe weekly is enough