Another scoop by Mark Gurman:
Our sources say that Apple is targeting 2.5 hours of “heavy” application use, such as processor-intensive gameplay, or 3.5 hours of standard app use. Interestingly, Apple expects to see better battery life when using the Watch’s fitness tracking software, which is targeted for nearly 4 hours of straight exercise tracking on a single charge.
I’d say this is on the lean side of the acceptability spectrum. It should be enough for many owners, but I can imagine the battery dying out before bedtime for more heavy users.