I have had my ipad for a year and use it ever day. Haven't noticed any decrease in battery health. The only battery problem I ever had with an Apple product was with an older Ipod classic. The one I purchased five years ago had the battery die out after about 5 months of daily use. The battery was replaced for free and I still use it, although the battery is now starting to die down.

I would say 4-5 years is probably a good guestimate of useful life, provided you let the battery deplete often and don't always have it plugged in. You really want to drain it completely at times and don't always charge it at half-cycle just so you always have a full charge.