I'll say it depends on water conditions but here they zoellers typically see 25 years or better.

thanks for the reply on my thread http://www.houserepairtalk.com/f33/sump-pump-experts-please-read-6874/
$1200 seems awfully a lot for replacing the main pump, adding backup pump and with a marine battery... But what do I know about how hard it is to install.
How long do Zoeller last? I am expending it to last at least 8 years? If so i'd be $150 a year for $1200 spent... otherwise, I might as well install a cheapo and replace it every other year at the same price.
your thoughts?