Well, it depends, when there's a game that I like on the 3DS, it can be 3+ hours a day (but it'll never exceed 5 hours/day), unless the game itself is really short.
Though recently, it has been a while that I haven't touched my 3DS (2-3 weeks), so, she's gathering all the dust in my room (not many interesting games recently, all of my 3DS games have been beaten, and the hype train will reach its destination only at the end of the year...).