In the UK, 3 years is maximum...although obviously you'll know if your'e due a replacement before then. You should make sure that you check your kit every time you climb (before and after) for wear and tear (particularly webbing).
As a beginner, I would think about getting a Weaver harness. They may be a bit heavier than the new "Technical" harnesses, but they sure do make you feel secure, they are incredibly robust and they jangle nicely when you walk!
They also have an inspection log attached to the belt, which is a good memory jogger. Try to get the one with the extra wide back support, you will thank yourself later! Oh, and they last like, forever.
DG