For notebooks and such, there are generally two types of batteries - 3-cell and 6-cell (or 300 cycles and 1000 cycles respectively).
In my company (which is not Apple but is one of the top PC makers in the worlds), we offer the 3-cell on most of our consumer notebooks, and they come with 1 year warranties for both unit and battery. That means your battery is liable to start losing effectiveness after 1 year but if you get in before the 1 year mark we'll replace it (assuming no abnormal CID). We don't do this to screw the customer, we do it to keep commodity costs low so ppl can buy $299 notebooks.
Our commercial notebooks get the 1,000 cycle batteries and come with 3 year warranties. Commercial clients have more rigorous standards. A company that purchases 4,000 of our units has a big IT department, and they don't want to be replacing employee batteries every 12 months; and after 3 years, they are very likely to just upgrade all their units to the latest and greatest anyway.
So, long story short (too late!), the battery in the apple watch is pretty amazing given its size and that it's a consumer-oriented item, not a commercial-oriented one.
Your typical 1,000 cycle notebook battery is the size of a shoe. This is the size of quarter (or half dollar?). Amazing.
(Oh, and of course the Apple Watch is a Watch not a notebook, but it is very much a computer...)