The Prompt Payment Act sets the clock the government has to pay you on, generally about 30 days from a proper invoice and acceptance of the work, though the exact timing can shift depending on the contract and how the invoice was submitted. If the government misses that window, interest accrues automatically; you should not have to fight for it. Late payment is usually a paperwork or routing problem rather than a refusal to pay, so the fastest fix is often just tracking down where the invoice actually got stuck.

In a sentence

"Payment is due within 30 days of receipt of a proper invoice." That clock is a legal deadline, not a courtesy, so if it passes with no payment, interest should already be building even before you have to ask.