I look after servers at work and replace the drives every year. I overlooked one set earlier this year and one of a pair failed. It was only 14 months old and was a SCSI very expensive and supposedly reliable type.
These units run continuously, so for home use the life span will be longer. I have one at home (6gb; massive at one time) that is seven years old and still going strong, though it is only used for occasional data storage.
The best plan for data recovery is to backup and backup, then backup again. Don't forget to backup, by the way. Did I mention backups? Backups are so cheap, and your data may be very valuable. If your business would grind to a halt if a disc failed then you should have more parallel discs and multiple backups, both on and off site.