Question
How long do I keep the papers about a repair done by a neighbour?
Keep the papers about a repair done by a neighbour while the thing they describe is still insured, and for a year after that. A receipt is worth most at the point where nobody can remember the year any more.
Work done as a favour still leaves a record, as long as somebody writes down what was done, when, and with which materials.
A repair that was paid for has an invoice. A repair that was not has a date and two people who remember it, and a note written the same week keeps that.