I rarely play on a portable for longer than 30 minutes at a time, and certainly not more than 2 hours. 3.5 is plenty for me, and I for most people without them realizing it. Though I'm not going to get one anytime soon either.
Honestly, what matters more to me regarding battery life is its long term survival. I mean many rechargeables have a maximum life of X hours, but after a month or so, suddenly it becomes more like 0.75X hours, then after a year, it's like 0.4X.. What I want is for the maximum battery life to NOT deteriorate over time. I've tried draining batteries completely before charging em, charging them all the time when not in use. Never having them plugged in when fully charged, all that. It still doesn't matter. Fact all rechargeable items have a nasty habit of NOT having the battery life they claim after less-than major usage. (back up a decade or so, and there were rechargeable batteries named "pure energy", which hilariously went from reasonable battery life to like... half hour after only like 4 effing charges, they were bull).
I have no idea how the 3DS fares in this category, and wont pretend to know, but I wish people would talk about THAT more than theoretical lab-tested 1-time use situations...