Mainly depends on the flat size and how often ppl clean. At my parents place you notice the cat smell only in her room… You can guess the size of that place, as the cat has it’s own “40 m^2 apartment” to sleep.
If you are in a small place and the cat always lies in the same corners is clear that they smell without washing. It’s like wearing the same hoody for months without washing.
My wife and I get two cats tomorrow for our 60m^2 flat. Let’s see how that pans out.