I have a Baytrail based Chromebook running GalliumOS (Ubuntu + XFCE) that is good for about 10 hours of usage (in an 11 inch form factor, a larger laptop could accommodate a larger battery). Apollo Lake is rolling out now and should be even better (~30% performance increase should lead to more time idling with the same workload). I'd expect firmware/software support for running generic Linux on it in 6 months or so.

Probably wont hit 20 hours, but I could imagine 12-14, which is pretty respectable.