Even if you're queued for a dungeon, you can still do other things so going AFK is no excuse even for that. Besides, you still have to be there to accept when the window finally pops up.

One hour. One hour is enough time to start on RL responsibilities before checking on the computer. If it takes longer than that, you're probably better off logging out and coming back once everything is taken care of. What about people who set up macros to get around any AFK timer? They're botting and should be removed just like any other bot. What if they've just set something against a button so it's constantly being pressed? That's trickier, but still cheating around a system designed to make the game a little more playable.

It's not like people who are away for 10+ hours will miss out on anything by being moved back to the character select screen. They weren't even there to see anything happen.