I just saw in the news how the Opportunity rover on Mars just entered into its 10th year of operation, with what its NASA operators say is still remarkably good electronic and mechanical health.
That's 108 months. Both it and it's twin rover, Spirit (which sadly stopped operating in March 2010) were expected to survive only for a three month mission. It was thought both would become covered with dust during the Martian winter and run out of juice — but they didn't.
To do the math for ya, Opportunity has lasted 36 times longer than its original mission. And to put that in perspective, given an average life expectancy for Americans born recently of 78 years, that'd be the equivalent of surviving 2808 years — and still being reasonably healthy.
We've definitely gotten our science-dollars worth out of those little rovers.