Hello,
I assume you are asking, because you are considering the purchase of a new iMac? There isn't an official life expectancy for drives due to usage patterns, environment, normal failure rate, etc. All drives fail, some faster than others.
That said, SSDs are more reliable than mechanical hard drives, and the Fusion Drive is a hybrid of both. For most modern drives, you should get at least be guaranteed 3-5 years, which is the normal warranty from most manufacturers.
Hope that helps,
C