That's assuming a 60-minute workout, using apps throughout the day, and the GPS model (which is slightly more energy-consuming anyway). The workout is really what'll get you; those switch the heartrate and other bio-monitoring from passive to active, which greatly increases battery drain.

If you use your Watch more passively, all-day performance is not out of the question. It all depends on your individual use case.

Besides, 18 hours should be sufficient since you ought to be getting more than 6 hours of sleep a night anyway. Somewhere in that offtime is when you can charge. I do it overnight, but plenty of people charge during their morning routine (shower, etc.) and say it charges more than enough to last all day.