The mainframes I used to work on ran for a month between reboots and uptime was 99.97% and this on a system that wasn't mirrored, didn't have Raid or dual PSUs etc.
Our record was 10.5 months without a single second of user downtime and then the failure was a harddrive so if we'd had mirrored drives that wouldn't have stopped us either.
Now where's that ladder I want to get down from my horse.