The Article Processing Fee (APF) is due immediately after acceptance. We usually expect the payment to be made within 30 days. We will start sending weekly reminders after 2 weeks after acceptance. It is in the authors' own interest to process the payment as soon as possible (ideally by credit card) because we can only proceed with copyediting and production when we have the APF payment on file (see also My paper has been accepted, what are the next steps?). Any delay in terms of payment will directly translate into a delayed publication.
If it takes longer than 30 days, then please keep us in the loop.
After 3 months (without communication) we reserve the right to deem the paper as withdrawn and will add all authors (including co-authors) to an internal blacklist, which means that we will not consider further submissions until the APF invoice for the accepted paper is paid. Even when the paper is withdrawn or not published for reasons that are not the responsibility of the publisher, the APF invoice remains valid and we expect authors to pay.
For payments after 6 months it is at the editors' discretion to decide if the paper can still be published as originally accepted or if changes are required or if the paper is no longer acceptable for publication.