This thing ate my reply, so here I go again.
MTBF is not how long the device will last. It is the average amount of time a device will last before something unexpectedly kills it.
So, a MTBF of 1,000,000 hours does NOT mean the device will last a million hours. It means that if you have a million of them, one will fail EVERY hour.
This number is really only important for things like large data centers which might be running 1,000 or more drives. For example, if you have a MTBF of 1,000,000 hours, and you know you've got 1,000 drives, then you can expect that every 1,000 hours (41 days), a drive is going to die.