Enough to reconstruct every payment and deduction: gross amounts, the materials element, the deduction, verification numbers for each subcontractor, and copies of the statements you issued. Keep them for at least three years after the end of the tax year, though we would keep them alongside your other records for the usual longer period, since HMRC compliance reviews on CIS are common and go back further than people expect. The material weakness we see most is the materials split — a contractor who cannot evidence why an invoice was treated as part labour and part materials will find HMRC treating the whole thing as labour and charging the deduction that should have been made, plus interest. Get the split onto the invoice, not into a conversation you half remember.