I agree with Gruber that the Watch will most likely not have the every-two-year buying cycle that Apple sees with the iPhone. I think the buying cycle will be between the iPhone and the iPad. I still have my iPad 2 and since it generally stays in my apartment, it’s rather unscathed and can do what I use it for easily. A Watch on the other hand will take a daily abuse, perhaps even more since it’s not in someones pocket.
I predict Apply Watch buyers will buy new every 3ish years. iPhone will remain every 2, laptops and iPads will be every 5ish years.