My first Android phone lasted 3 years. Actually it still works fine, but it was far enough behind the curve (especially in storage) that I decided to upgrade anyway.
My current phone is 4 years old, and beginning to show its age, largely in terms of reduced battery life. That said, both of my iPhone-owning brothers replace theirs after 2 years because they say the battery degrades badly after 18 months, and my daughter's iPhone 4s was being charged several times a day by the end of the 3 years she used it, while I only became watchful of the battery after 3.5 years, so I'd say it's done well by comparison. I also know a number of people using similar age Android handsets, and still see others using the same model as me most weeks.
As said above, it really depends what Android you buy: they are not all the same.