I had a Masport with a Briggs & Stratton motor that I bought in Canberra in 1993 and was still running like new two months ago. It had a bit of rust, but not too much, and was in pretty good condition.
Over that time I probably used it every one or two weeks (maybe a bit less in winter), cleaned it after every couple of uses*, sharpened the blade once a year, had it serviced (maybe every three years or so), and replaced the pull cord, spark plug, and an air filter once each in that time. Only sold it because I moved into a house with no lawn...
* It also had three really thorough cleans, as after buying it in Canberra I moved to Wellington, then to Adelaide, then to Blenheim, and lawn mowers get a lot of scrutiny from the bio-security guys when your international container is getting unpacked. One of them told me that if he sees a lawn mower on the manifest he always checks that first - it's a great indicator of how much care people have taken with their stuff, and therefore how the rest of his day is going to go.