I asked WD how long a lifespan their drives were engineered for, and they told me 5 years. Didn't differentiate among types or models.
My experience has been that if WDs make it past the first month, they usually last about 5.5 years, but those that go beyond may last indefinitely. (I have some with 11+ years on 'em, 24/7/365.)
Back in the day I dismantled some dead SCSI drives, and found inside the exact same HD as the IDE variety of the same brand. Difference was in the interface, not the guts.