I'll upgrade when they make compelling additions and finances allow. For my phone, that puts me on the S cycle.
iPhone 3 -> 3GS largely because it brought me Siri
iPhone 3GS -> 4S largely for the fingerprint reader
iPhone 4S -> 5S for background location services (if I remember correctly. Or do I have the 4S and 5S the wrong way around?)
Now I'm in the habit of updating every two years, so I'll probably continue. The only thing in the 6 and 6+ that I want is the barometer. I'm looking forward to seeing what the 6S or 7 will bring.
In short, I'm a sucker for new tech, so I wouldn't be surprised if I end up updating my watch every 2 years and I reckon the battery will be at the end of its life after 3 years, so I might as well update it then if I haven't before then.