There is an excellent free app called Battery Life that will tell you all the information about your battery. Apparently batteries can be charged about 300 times during their life. This app will tell you your battery wear life. A 100% battery life on my iPad mini 2 is really on 93% which is a low battery wear level of 7%.
My iPhone 4S has a battery wear life of 12% which is average. My iPhone is almost 6 years old. To have a battery replaced where I live costs $40.00 for an iPhone 4S and $100.00 for an iPad mini. I haven't had any experience with having to have a battery replaced yet.