First of all Interstate batteries are top of the line!
Second, they last around 5-7 years. I have a 8 year old one in my hot rod right now (works great).
Third, you don't need to start your car to keep the battery topped off, get a cheap charger and use it once every 3-4 weeks.
Fourth, take your car for a decent ride (20+ miles) park it and see if it starts a couple days later. If not, get a new Interstate and repeat step 3.