The short answer: her whole life
Medical records are not like household bills, where a sensible shredding date arrives. Your cat's history stays relevant for as long as she lives, because questions that arise late in her life — about a procedure years earlier, a reaction once noted, a vaccination sequence from kittenhood — are answered from exactly the paperwork owners are tempted to throw away. Keep the core file for her lifetime as a rule, and treat any urge to thin it as a prompt to organise it instead.
The core file is smaller than most owners fear. It means the vaccination history, microchip details, neutering confirmation, notes of any surgery or hospital stay, and a running summary of ongoing conditions and the care they involve. That set, kept complete, answers nearly every question a new vet, a boarder, or an insurer will ever ask.
What can be thinned, and what must never go
Routine paperwork accumulates faster than core records: duplicate appointment slips, superseded estimates, and repeated printouts of information now summarised elsewhere. Those can be reduced once you are confident your clinic's own file and your core set between them hold the substance. Ask the clinic at a routine visit what they retain, so your thinning is based on knowledge rather than hope.
Some documents should simply never be discarded while your cat lives. Anything proving vaccination status, identity and chip registration, or a significant procedure falls into that group, as does any note recording an adverse reaction — a detail that could change a future decision and that no one will remember accurately years later. If in doubt about a particular document, keep it; storage is cheap and reconstruction is often impossible.
Records at the moments they matter most
The value of a lifetime file shows at transitions. A new clinic after a house move starts from your file rather than from your memory under stress. A boarding stay, an insurance question, or an emergency visit to an unfamiliar practice all go better when the history arrives with the cat, complete and in order.
And if your cat is ever rehomed, her records should travel with her as a matter of course. Her history belongs to her care, not to your filing cabinet, and a new owner starting with a full file gives her continuity that a fresh start otherwise loses. Keep your own copy of what you pass on, in case questions come back to you.
Keep them findable, not just kept
Retention fails in practice through disorder, not destruction. A lifetime of paper in unsorted boxes is barely better than no file at all when a clinic needs an answer today. Keep the core set together, in order, with the most recent summaries on top, and review it at the same time as her other yearly paperwork so gaps surface while they can still be filled.
Holding that lifetime set in her KinsLe Pet Passport gathers the vaccination history, chip details, and procedure notes into one portable packet, so the record you have kept for her whole life is the record you can actually produce, complete, whenever it is asked for.