Usually about 2 years, for example I got the iPad Pro and then the second one (2015 to 2017) before that I had the Air 2 and before that one I had my very first iPad which was the iPad 3. However I may be breaking with tradition this year IF Apple do a design change and put Face ID into the new iPad Pro which is rumoured to be taking place, if that happens I will get this years iPad as well and pass my current iPad Pro onto a family member.