Mike
Me and Denis live in Worcestershire. We don't walk on our upholstery

You must have some funny habits up in Hull

As for a generalisation though Mike, I'd go with you "When it's dirty" or better still, a little before the soiling level shows through.
Denis. You need to ask questions.
How old?
How many times has it been cleaned and how?
When was the last time?
If answers are unsatisfactory, suggest an audit (by yourself) at 3 months, then 6 months etc. until you are able to determine a fair interval.
It's not inconceivable that you could be cleaning those that need it at, say, six months, and everything at twelve months.
Safe and happy cleaning

Ken