Hopefully you were running at least a RAID1 or RAID5 so that your users weren't impacted by the hardware failure!
For us, if there were two failures in a 12 month period, there's some serious problems going on. The hardware that we use on our machines is NEW hardware however, so that really helps cut down on possibilities. If you're using recycled hardware, or older models, you run a higher risk of hardware failure.
Usage will play a part in things. The general school of thought is that a home computer hard drive would last between 3 and 5 years (I have some drives still running that are 10 years old and run daily, just not 24 hours).
Another thing to be aware of is that in general, hard drives are not designed to run 24x7 with massive read/writes for years on end. If you're looking for stability when it comes to drives, check about investing in "server grade" hardware. There's actually specific drives designed to run with the high stresses of servers.
When all else fails - mirror
A RAID 5 is good, a RAID 10 is better, and a RAID 50 you'll not have to be concerned about any failures at all. You can have several drives fail at the same time, and you won't even bat an eye at it!
As always, the logs are your friends. Before a server fully tanks, usually there's some sort of heads up (not always, but many times you'll get a warning). If you notice SMART errors or anything else related to failures, then you need to address that. Some server owners disable the FSCK after "X" bootups (usually 10 boots), but this is a very bad thing to do. the FSCK is going to detect the integrity of the drive and determine if there's any errors found. Not doing this on a semi-regular basis, or at least after every "X" boots, can lead to issues down the road.
If you're replacing a drive within a matter of months of each other, 9 times out of 10, you're using recycled hardware. Install NEW drives. (hardware degradation starts the moment they spin up, but USUALLY they don't fail right out of the box)
Final advise regarding harddrive selection - don't buy the cheap ones with a 1 year warranty - get one with a 3-5 year warranty. Reason? Manufacturers don't want to be replacing drives so they use quality parts (usually). A 1TB drive for $60 is generally not made the same way a 1TB drive for $300 is made.