Ok, so I just looked at the Vita battery stats and it's 3-5 hours, if your offline, no 3G/Wifi, default screen brightness, headphones, no bluetooth and a non-system taxing game.
So chances are that if you turn on any of those things, while playing a pretty intense game, it could take you longer to charge the unit (2hr40min) that it will for you to completely drain it. And that is with a battery that is almost 1000 mAh more than the 3DS (1300 mAh).
That means that in a realistic play setting while on the go, you are looking at anywhere from 1.5hrs to 3.5hrs of Vita usage between full charges.
edit: And then on top of that, the memory card isn't included (in Japan so far) and cost alot more than a standard SD card.
Memory card 4GB December 17, 2011 ¥2,200 ($29)
Memory card 8GB December 17, 2011 ¥3,200 ($42)
Memory card 16GB December 17, 2011 ¥5,500 ($72)
Memory card 32GB December 17, 2011 ¥9,500 ($124)
Considering that 2200 yen for a 4GB card is close to $30 US dollars, where you can get a 4GB SD card for $5-$10, I'm not sure how well that is gonna go over.