Wait until the payment term you stated on the invoice has actually passed - not a day before. If you did not state a term, UK law treats 30 days from the invoice (or delivery, if later) as the default. Once that date has passed, the invoice is officially late and you are entitled to chase it and, in business-to-business trade, to charge statutory interest.
What counts as "late"
An invoice is late the day after its due date, full stop - there is no unwritten grace period, even though many people wait a few extra days out of politeness before saying anything. That politeness is optional, not required; you are entitled to follow up the moment the due date passes.
A sensible chasing timeline
Due date + 1-3 days: a friendly reminder - "just checking this hasn't been missed", assuming good faith
Due date + 7-10 days: a firmer follow-up referencing the original invoice number and due date directly
Due date + 14-21 days: a formal notice stating the amount owed, days overdue, and that statutory interest may be added
Due date + 30 days: consider whether this needs escalating - a formal letter before action, a debt collection service, or (for B2B debts) the small claims court
Should you wait longer for a good client?
A single late payment from an otherwise reliable client is worth a softer, faster nudge rather than treating it as a dispute - it is very often a genuine oversight, not a refusal to pay. But "they are usually good for it" is not a reason to let weeks pass in silence; a quick, friendly check-in costs you nothing and often resolves it same-day.
The single biggest lever you actually control is not how long you wait to chase - it is how clear the original invoice was. A due date printed clearly on the invoice, not just mentioned verbally, is what makes a follow-up feel like a reminder rather than a confrontation.
How long should you wait for an invoice to be paid before chasing it?
Wait until your stated payment term has passed (30 days is the UK legal default if none was agreed), then chase immediately - there is no required additional grace period once an invoice is genuinely overdue.
Is 30 days the legal maximum for invoice payment in the UK?
Can I charge interest on a late invoice?