Buy With Confidence is just the source of the info.
It is 14 days for all domestic customers, you should be issueing contracts for any job over £42 to be fully compliant, and if you don't comply the legislation is fully open ended as it is criminal law, so a customer could demand money back in 5 years time and you haven't a leg to stand on if you don't have a signed contract.
Only exceptions are emergency call outs.