How often does the frugal person replace mattresses?

I noticed what people spend and how often are all over the place. People are spending like $3k or more! I just got a couple of hybrids for my spare bedrooms (one was $250 the other $600 as I sleep on it often). Another spare room set is over 25 years old and still going strong. The one in master bedroom is 4 years old and shows no signs of wear.