Enduring declaration
A Gift Aid declaration can be limited to a single donation, or it can endure. Most charities use the enduring form, and most Gift Aid problems trace back to its wording.
An enduring declaration typically covers donations made in the past four years and all future donations until the donor cancels. It must contain the required elements and identify the donor well enough for HMRC to trace them — a full forename, not an initial.
Keep the declaration for as long as you are claiming against it plus the retention period. See Gift Aid declarations and audit trail.