Probably a very, very long time. Watches have been around for centuries, and now that we have watches that do so much more than tell time, it's safe to say these are here to stay for a good minute as long as they continue to provide useful tools that people find value in.
The only thing that could potentially shift everyone's attention away from the Apple Watch are the rumored glasses but who knows when those are coming, what all they can do, and how accessible they will be to most people. Much like the Apple Watch, I think these glasses will see a very gradual adoption by the general public. I imagine they won't be cheap and many will abstain from buying them for a number of reasons. So for this reason I don't think the Watch is any danger of going anywhere. In fact, Apple will probably find the way to make both products depend on each other (ie: Apple Watch requiring an iPhone), ensuring the Watch is here to stay.