Since I don't have a Switch (yet), I wrote down Digital Foundry's results for Zelda BOTW with full and half brightness; with and without different power banks:
This is only a place holder until I get my Switch, then I will test it with different games and more different settings.
Since one of the power banks of DF's test is similar to mine (both 15 aH / 55 Wh), I made long-term tests with most of my devices. No different settings or games... always the worst-case scenario (full brightness with the most demanding game):
Two exceptions: I chose "GTA Vice City Stories" for the PSP because I didn't want to kill my UMD drive. And I lowered the brightness for the 3DS XL and turned 3D off because that handheld transfered the energy too late while on maxed settings.
So with a moderate power bank and if the cable to the handheld doesn't bother you (many play handhelds with a wired headphone), you can run any Sony or Nintendo handheld (besides the Switch) for 24 hours and more. A smaller sized power bank like the "Anker P'Core" recommended by Digital Foundry should still be enough to run any Sony or Nintendo handheld (besides the Switch) for over 20 hours.