If you are not planning to sit in Cafe Rudolfinum before the concert, it is ideal to arrive 30 minutes in advance. You will have enough time to put your things in the cloakroom, find your place in the hall and take a peek at the programme.
If you arrive early, you won't be bored. Before the concert you can visit Jiří Bělohlávek's Listening Lounge and browse the Rudolfinum Shop. Before the concerts of the A, B and C subscription series, there are also pre-concert talks, where a music publicist, often together with guests from among the performers, will introduce you to the programme of the upcoming concert.