I know Payroll has a time frame and wondering if AP does too. Is it just up to the company to establish their own best practice for such an instance?
There are state laws governing the timing of final wage payments. Similarly, how quickly final expense payments must be made is also determined by the applicable state.
California's Labor Code 2802, for example, requires that reimbursable expenses be paid no later than the normal expense payment cycle, just as if the employee had not been terminated, although expenses do not have to be paid as quickly as final wages do.
Other states are not as clear, so you will need to research this with the appropriate state's Department of Labor. Brief research seems to indicate that expenses should be paid either within 30 days, or, like California, at the same time they would normally have been disbursed had the individual still been employed.
This listing should be helpful in locating the appropriate state agency to contact: http://employment.findlaw.com/legal-help-and-resources/state-labor-agencies.html.