I normally update my iPhone every two years. I've had the 4, 5 and now 6. I update my iPad every three years. My iMacs have been lasting about 4 to 5 years. I also hand these things down. My 2007 iMac is still being used by my Mom (and she also has my iPhone 4) and my iPad 1 is getting some use by my Sister and her household. My GF has my iPad 3.
We shall see about the watch. I suspect that the tech will improve enough that if I'm using the watch every day it will be worth it to me to upgrade at least in two years.
Incidentally, now that I'm no longer on contract, I may switch to upgrading the iPhone every year. Yes that is a very expensive thing to do, but since the phone is super important and I use it all day, it is still a justifiable use of my money. The watch may be the same thing as just too useful not to upgrade.