In Bridge, when should third-hand defender overtake partner's honour?
Agree with what Forget I was here said about the bidding. And also what they said about counting. What I want to explain is that I think the defence will prevail, if North lets the heart Jack run to declarers Queen. If declarer then runs the spades, North can pitch a diamond. 


  Then if declarer exits in diamonds, North will get in, and now play Ace and another heart. This promotes the 13th heart, and endplays declarer.


Observe that it is not good enough for North to exit with a low heart, for then declarer can endplay him with his small heart (this was already explained in the other answer). In a way playing the Ace and a third heart removes declarer's exit card of a low heart.

So this time it was essential for North to wait the ace. The rule of thumb of using the ace to win opponents honors would have served well. On some other hand it would be more difficult but now it is rather likely that declarer has KQ hearts.