Upgrade cycles always start off short and get longer with every generation. If you had an original iPhone, by the time the iPhone 4 came out your phone was basically obsolete. However, I know lots of people who have an iPhone 6S that's still going strong with no issues, and it's at the same place in its life cycle.
My Series 0 watch was already showing its age 2 years in, but I'd expect a Series 4 to last at least that long if not longer. It's still early days though, which is why I chose aluminum for my Series 4 when I previously had Stainless Steel - if I plan on upgrading in a couple of years, I don't see the point in paying twice as much for a steel model. By the time the Series 6 or Series 8 are out, though, I might change my mind.
My Series 0 watch was already showing its age 2 years in, but I'd expect a Series 4 to last at least that long if not longer. It's still early days though, which is why I chose aluminum for my Series 4 when I previously had Stainless Steel - if I plan on upgrading in a couple of years, I don't see the point in paying twice as much for a steel model. By the time the Series 6 or Series 8 are out, though, I might change my mind.