I'd say it would last 7 years, but something cooler and better and newer will come out and most will end up replacing it sooner. Again, my estimate depends on how much you use it...
It will probably set your house on fire just before it pegs out, destroying any evidence that it failed.
The 'server grade' (If you invest in really bargain-basement servers or Apple 'Pro' gear, sure) HDD's lifespan will probably be significantly curtailed by how hot this thing runs.
Thanks everyone. I am thinking about weather or not it's worth it to get this add on. I hear everyone talking about Drobo but it's too expensive for me.