Ask around and you will be told to change the cylinder at every changeover. It is simple advice, it is easy to give, and for most properties it is money spent on the appearance of security rather than on security.
Here is the version that holds up.
Change it, without hesitating, when
A key has not come back. Not “they say they lost it”, not “it will turn up”. A key is unaccounted for, so every key that exists is now untrustworthy. This is the only rule that matters and it overrides everything below.
A long-term tenant has moved out. Someone has lived there for a year or more. You genuinely do not know how many copies were cut in that time — for a partner, a cleaner, a relative, a friend who was staying — and none of that required your permission.
The keys had the address on them. A fob with a street name or a flat number turns a lost key into a location. Change it the same week.
There has been a dispute. Whatever the rights and wrongs, a tenancy that ended badly is not one where you want to be relying on goodwill about key returns.
You have just bought the place. Every external door, straight away. Previous owner, agent, builder, cleaner, the neighbour who watered the plants — you have no idea and no way of finding out.
Do not bother when
It is a short stay and the keys came back. A week’s holiday let where the set was handed over and handed back does not warrant a new cylinder. Doing it fifty times a year is a real cost and it buys almost nothing.
Nothing has changed except the calendar. A cylinder is not milk. It does not expire.
The middle case
Short lets where keys go through a key safe or a key holder rather than hand to hand. Here the risk is not the guest, it is the accumulation: a code that has been shared in twenty booking messages, or a key holder who has changed twice. Reviewing that annually is more useful than changing the cylinder monthly.
A note on keys themselves
When a new cylinder goes in, it comes with the manufacturer’s keys and that is the set you have. Additional copies for a changeover are a key-cutting shop’s job — not every locksmith supplies keys, and this one does not. Worth knowing before you plan a handover around it.
The thing landlords actually get wrong
Not the frequency. The timing. A cylinder change is a twenty-minute planned job at normal rates if you book it in the gap between tenancies, and an out-of-hours call-out if you remember on the evening the new tenant is standing outside with a suitcase.
If this is where you have got to, the page you want next is a property that stands empty.