Provision first, write off later
Accounting distinguishes two steps. A provision (or expected credit loss allowance under IFRS 9) recognises that some receivables probably will not be collected, while you keep chasing. A write-off removes a specific debt from the books once you have concluded it is irrecoverable.
Our view: most SMEs should review the AR ageing every quarter, provision specific debts over 120 days where recovery is doubtful, and write off only when there is clear evidence: the customer has ceased trading, legal advice says pursuit is not worth it, or a settlement has been agreed for less.