Does it take credit cards or euro? How long does it take?
Sorry but I can't recall whether plastic is accepted. This museum is NOT to be missed and I'd strongly recommend not rushing through nor visiting The House of Terror at the end of a long day of sight-seeing. There's an audio guide or, perhaps, guided tours. I'd estimate no less than 2 hours; longer if you're keen on the topics of politics, WWII, the Cold War, etc. since you will want to linger.
We visited both the House of Terror and then Opera House one afternoon; however, I'd leave plenty of time for refreshments at the Cafe Calas (next to the Opera House) - a good breather between the two.
Yes, they accept credit cards. I agree, I'd reserve two hours for this. Note that the museum is closed on Mondays.