How often can a landlord inspect a property?

As often as is reasonable, with at least 24 hours’ written notice and at a reasonable time of day. In practice that usually means every three to six months.

The tenant has the right to quiet enjoyment, which means you cannot let yourself in whenever you like, even though you own the place. Turning up unannounced is a trespass, and repeated visits can amount to harassment.

Notice is not the same as permission. If the tenant says no, you cannot simply go in, and the answer is a conversation rather than a key.