I used to get 8-9hrs on macOS and about 6-8hrs on Ubuntu on the same Macbook. I'm sure Linux can do much better on non-apple hardware that is more or less open or well known in the Linux community. The problem with Linux and power savings is that a lot of the proprietary hardware manufacturers don't release specs that make writing drivers easy for obvious reasons so Linux open source driver implementation is never as good as macOS or Windows. Dell's XPS13 developer edition has been doing quite well in the last couple of years. If it picks up more steam, we could see Dell or Intel write proper open source (or closed) drivers to support the hardware better.
I'd get about 9-10 hours on a Macbook Pro simply lazily web browsing. This is theoretically reported battery life, however.
I never actually let it run down to 0. It's possible the last 10% would have vanished in 30 minutes instead of ~2 hours like it claimed.
I can certainly believe a Linux machine can get 14hrs. It's just a matter of having a bigger battery, and a processor that will step-down its speed when its not needed.
