With a series 3, even the cellular version, the battery will easily last 2 days for most people. Even with heavy use I usually have over 50% battery left at the end of the day...
I've had to charge my series 3 every night. I guess not having it paired with a phone somehow uses more battery power? Technically, I reset my wife's phone with my sim+apple id, paired, reset back to wife's backup and sim. So it has no real phone to pair with, though I guess maybe that causes it to use cellular a lot more than most.