I've trialled mSATA, SDXC and Compact Flash.
16 hours is easily achievable using CF and a "fat" back that fits a 1900mah battery. Rockbox provides a plugin called Battery Bench that allows you to benchmark how long you can continuously listen to music for.
Having recently switched from mSATA to SDXC I am yet to run a benchmark for SD. From recent usage I would guess 12hrs is fairly easily achievable.
EDIT: bwoodward has benched 60hrs of playback with a 5th gen Video and SDXC below.
Depending on the device you use, you can squeeze out > 40h of pure listening time.
> Music devices are like cameras, the best one is always the one you actually have with you.
Hehe, nice comparison. But the iPod is not about having the BEST sound quality. It's more about being decent enough in every feature it has. Pretty sure iPods can't compete with the Hi-Res DAPs like Fiio M11 Plus, Astell&Kern, iBasso and others in terms of sound quality. But the user experience may still be better, depending on your personal taste.
How many?
> Amazing how every comparable device made today is so much worse.
Music devices are like cameras, the best one is always the one you actually have with you.
That said, nothing will compare to the tinny sound of a Rio PMP processing 128kbps mp3s that have been recompressed to 32kbps. Sounded like angels in your ears were playing the instruments.