All depends on how they are cared for.
If you are constantly installing, un-installing, it - etc, then the wear will be greater.
Keep it clean, make sure it's mounted securely and that all connections are tight and covered, and they can last quite a while.
My current HU (Sony CDX-M630) in my T-Bird is pushing 2.5 years without issue and still does everything I need it to.
I have an Alpine(Can't recall model # presently) going on 4 years old in my Cougar Show Car. -Also without issue, however, it will be upgraded this year as it no longer can provide everything that I need.
Also have a Sony EXCD-21 (Circa 1996- IIRC ) that is still working flawlessly, and has been without issue the entire time.
I tend to border well beyond that of obsessive-compulsive in maintaining things, however, it just goes to show you that the life expectancy of a product largely depends upon its owner.
Car Audio/Performance becomes addicting. Really addicting.
Expense wise. Yeah........ //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif