A few hours
That's how long it took The Boy to conquer this 49 piece 2x3 foot puzzle. I'm not kidding either. I gave it to him last night and handed him the pieces the first time he put it together. This morning he was able to put the entire thing together on his own. He then informed me, "You should probably get me a Thomas puzzle from the store."
Hopefully I can find a more complicated Thomas puzzle.