In reply to hikerman:
The website the other chap posted has full details. How long? hhmm that does depend on timings of your flight, connections etc. Depending on when your flight arrives in Oslo or departs you may have to stay overnight in Oslo. Mind Torp is nowhere near Oslo - although I do think you can still travel to Rjukan from Torp. Not so sure on how good the connections are though.
By car from Oslo to Rjukan is around 3 hours, maybe less or maybe slightly more depending on road conditions. Overall bus is longer due to connections etc.