I don't think I have ever seen that happened here in the UK. I seriously can't remember a single time it has happened. However, I have heard it is quite common in the States. Do you clap and certain scenes or have you been there when others have clapped? How do you feel about it?
I personally don't see the point in clapping. I understand clapping at a theater show or play or at a sports game, but the cinema is different. When you applaud someone, it's best when they are there so they know you are applauding them. When you clap at the cinema, they have no idea. You are basically just clapping at a giant screen.