It depends on what you're doing, but the SoC is relatively powerful for it's size, and the battery is relatively undersized compared to the chip. Anywhere from 2-6 hours depending on what you're doing, generally.
I manage to get decent battery life, somewhere around four hours in general usage (not gaming or compiling) with TLP and disabling boost on battery, along with a few other small tweaks.
The Air is going to be a very different product because it is produced in much higher volume. And it will have many more engineering hours spent on it to make things ‘smooth’, including having the speakers sound OK by applying DSP as this post discusses. The GPD will be much more hackable but have worse performance as a trade off
