To my knowledge, that is not natively built into FusionpBX. The most efficient way to create such a thing would be to set a variable in the variables dial plan such as, "set monthlyquota=2000" and then call a Lua script in a later dial plan that checks the v_xml_cdr table to total the minutes used that month against the monthlyquota variable. I don't think it will be particularly difficult to create, but I'm pretty sure it's not built in.
Keep in mind that this would be a fairly heavy call to the database for every call. You'll also want to avoid making this DB query when the call is a local, extension to extension, call too.