My general rule is never to applaud anything except at the end of a scene or act. One exception is Zerbinetta's "Grossmaechtigen Prinzessin" in Ariadne auf Naxos. Another is "Ah, mes amis" in La Fille du Régiment.
I went to see Bellini's La Sonnambula (for the first time) at Covent Garden recently and don't feel like seeing it ever again, especially when conducted by Daniel Oren, whose tempi were really soporifica. And the audience couldn't understand that in bel canto many arias have a cavatina and then a cabaletta (with sometimes a tempo di mezzo in between), so there was applause practically all the time.
I've also noticed that in live radio broadcasts nowadays there are increasingly bouts of applause in between movements of orchestral works, mainly concertos.