The only date that must be on a valuation report is the valuation date.
There is no prescriptive standard on the longevity, expiry or length of validity of a report. A report however does have to be dated so that the parties are aware of the data point. The Terms of Engagement may outline expiration, but not always. The Red Book Global Standard 2025 does cover revaluation where there is a previous report under VPS 4 part 2 (in 2022 Global Red book this was VPS 2 part 2) "Revaluation without re-inspection of real property previously valued" as it is recognized that time may pass from the initial valuation.
Some lenders or other third parties may have their own periods of time that a valuation may be acceptable to them and it is generally down to whoever is relying on the report who decides when the report's usefulness to them expires.
The RICS does not set dates for a valuation to expire. 'Extending' the valuation is a process that needs to be agreed with the parties who will be relying on the report. It may be that the lender just agrees to extend the timeframe . They may accept a 'desktop valuation' which builds on the original valuation.