It worked for 9 years and 7 months, whilst being planned to operate for 3,5 years, so beyond it's expected lifetime then.

And it would have worked for longer if it didn't depend on the fuel!

BTW, I doubt there's enough experience with space hardware to accurately predict it's life, especially since each machine is a one-off.

It would have lasted a lot longer if its reaction wheels hadn't failed. That was the main failure which shortened its remaining operation.

Kepler had 4 wheels, with 2 failing