There is a time limit for factoring companies that require a financed invoice to be paid within ninety days. In other words receivable financing is only for businesses that issue 30 day net term invoices and have a history of getting paid in a timely fashion. If the cash flow model of your business is not getting payment for work completed within a relatively short time (vs. quarterly or annually) then factoring invoices might not be a solution for you.
The reason is, the arrangement a factor has with its funding source usually is a leveraged institutional bank line. The bank lends money to the factor who then turns around and makes advances to their clients. When calculating the daily available credit to the factor, any outstanding invoice over ninety days is considered a non-performing asset and is deducted from the available credit line.
In some cases, a normal 30 day invoice will age over 90 days. In these situations a recently finished job with a new invoice is swapped for the overdue invoice, thus retiring the old one. But this should happen only on rare occasions or the factoring company will pay closer attention to your business model.