When should I applaud?  At the very end of a piece, no matter how many individual movements there are. The purpose of waiting is to maintain an unbroken atmosphere so that the piece retains its unity and that any spell the music has cast remains unbroken.  If you are worried about when to applaud, the safest course is to wait until the conductor has turned around to face the audience and everyone is clapping.

Why should I be quiet during the concert?  Silence is the canvas on which music is experienced, so for maximum concentration and enjoyment, the audience should be as quiet as possible.  In the sensitive acoustics of the church, even the slightest noise can be a significant  distraction. What would normally go unheard in daily life (whispering, humming, tapping, turning pages, etc.) is noticed by everyone around you. Of course, mobile phones and all other electronic devices must be silenced before the performance begins.