It varies depending on what the demand is for that night. For example, I imagine the Last Night of the Proms requires a lot of queuing.

The last time I went though I think almost everyone in the queue got in. The person who sells the tickets in the queue can normally tell you what your chances of getting in are.

My advice is to arrive 30-45 minutes before the performance, with a blanket to sit on and some snacks and just have some alternative things to do in South Kensington in case you don't get in