Obviously any response here will be pure speculation. But I would anticipate that the watch would yield a very similar cycle to iPhone and iPad (i.e. annually). I could see it being a bit more squishy than iPhone, because it doesn't have carrier contracts to worry about, but I think Apple beleives that 1 year has become a good development length (they've even moved OS X to annual releases). They'd probably do the same with Mac's, but are dependent on Intel.
I hate to sound unoriginal and like a broken record, but I'd anticipate pretty similar OS updates and support cycles to iPhone and iPad. So essentially 1 big annual update* and support Watch hardware for around the ~4 year timeframe. iPad 2 came out March 2011 and will likely be supportd through Oct of 2015, so that's 4.5 years. iPhone 4s came out in Sept 2011 and will be supported through sept 2015, so that's 4 years.
Of course that's not to say the Apple watch will stop functioning at the 4 year mark of course, but iPhone 4 just lost support last month. I think anyone who's walking around with an iPhone 4 either despeartely needs to upgrade or is the type of person that doesn't care about having the latest software updates anyway.
*alternatively, I could also see Apple taking a new approach to software updates. Giving minor updates periodically and not having a major X.0 release ever (or at least not advertising as such)