As others have said, 2.5-6 hours (and yeah, Zelda is 2.5 hours).
But that's the point: the original 3DS managed 3 to 5 hours. It got bashed for it pretty hard by the way. Heck, I got in on the action. But while the 3DS didn't get a pass for it at the time, the Switch sorta gets a pass. Its horsepower is state-of-the-art for $300 mobile hardware in 2017, whereas the 3DS was much more conservative and was the successor to the DS, a system that had great battery life. Also, the limitations of battery tech are even more of a bottleneck now than they were 6 years ago.
Basically: 3-5 hours was shit back then and certainly didn't live up to consumer expectations, but it's very decent in 2017. It's sad, but that's the way it is. Battery tech sucks. Hard to fault Nintendo for it. Other manufacturers would deal with the same issue unless they drastically cut down on graphics horsepower or put a massive, bulky battery in it.