I'd say probably around nine or ten hours? If you mean not including bathroom breaks and grabbing food from the kitchen. My sister and I did basically nothing but play New Leaf for about four months after the game came out.
Oh gosh, probably about 12 hours. That was back when the game came out years ago, though. I was young and it was the summer, I had nothing else to do and New Leaf was my way of talking to my "friends" any time I wanted. Even if they were cranky cats.
Probably between 8-10 hours, when I was doing my first paths. I spent all day long trying to make it look the way I wanted. Then I realized I reallly didn't like the paths I chose, so I went all over the town and got rid of them. This time around my paths took a fraction of the time.
When I was actively trading/cycling villagers for others, I could be online all day doing trades with a few breaks in-between. Nowadays I play for about an hour before moving on to something else, though I'll pick it up again eventually.
According to my Activity Log, the longest continuous time I've played ACNL since November 25 is 8 hours and 55 minutes. November 25 is when I started playing again, and I'm not looking at dates before that. However, my Activity Log also shows that ACNL is my most-played game ever, with 439 hours and 55 minutes since June 13, 2013. That dwarfs everything else that I've ever played.
Well, probably about 4 hours straight? I know my longest session was about that long, and I think that was either on my first day playing, or bug collecting in order to get the crown.
Though there's been days where I have the game open for the entire day and play for a while, then go online for a bit, then go back to the game, etc
8-9 hours when I was really into qr code designing and just kept designing and designing and designing until I noticed I'd been doing it for ages and really needed to take a break