nexus 7 will be obsolete before your battery is, most likely. 2 years from now you'll be seeing probably 80% of the charge when purchased...even then the battery life will still be decent enough, the charge times are rapid enough that you can plug it in for a short period and get multiples in usage minutes out of the minutes it took to charge. i.e. plug it in 10 minutes, and you'll probably get at least 40 minutes of usage out of that charge. so even if you go through more than one charge a day, there isn't too much downtime between recharges.
I look at the nexus 7 as a turn-on-and-forget device. the only thing you need to worry about is not dropping it or scratching the screen. it's not like other devices on the market offer anything better, especially at this price range.