This varies based on the service, but in the UK, the general rule is that you need to keep records for 6 years. We will let you know exactly what you need whenever a reporting period is ready to be prepared, but you if you would like to know in advance you can read our "What records do I need to keep?" post for more detail.
As a brief summary, keep a digital copy of all paper expenses at a minimum, e.g. using a Google Drive. Ideally, file any electronic invoices/emails into a single folder for easy retrieval. For personal taxes, keep all of your end of year employment documents (e.g. P11D, P45, P60), any end of year interest statements from non-ISA bank accounts, and a record of any income that is not taxed at source (e.g. rental income, sales of shares outside of ISA accounts, Sole Trader work, etc).