I'd guess they'd have a refresh cycle closer to Macbooks. Update internals every 18 months or so, but not the design. Maybe 3-4 years for design update? By then, they could have one with a much better battery in a thinner design, basically making it still have a one day battery. To get two full days in the same form factor would take a drastic change in battery tech I'd think.
MacBooks are updated based on Intels roadmap. The watch, like iDevices, has no such dependency.
If we've learned anything about Apple it's that they have no respect for people's past investments and making sure we get a nice long life out of them. On the contrary, they have a long track record of annual incremental and interesting (even innovative) updates to their mobile devices that dates back over a decade. They are so effective at this constant cycle of upgrades they are the envy of the industry. And besides the iterative design improvements, the constant increase in software sophistication almost ensures obsolescence within a few generations.
The watch will be no different than the iPod, iPhone and iPad that came before it in this regard. In fact, Apple is about to school the Swiss watch makers on how you get someone to buy a $600-$1000 watch every 2 years. Wait and see.