Couple of hours I think, but I could've spent longer reading things thoroughly - I was more focused on seeing things/taking pics & soaking up the atmosphere.
The interactive bit is about 30 mins, then you've got the exhibition. There were plenty of people there, but not enough to cause congestion anywhere, so I'm guessing they limited the numbers for the previews. Now it's officially open I'd expect it to be busier, in which case I would think you could be there longer to be able to see everything properly - no attempt was made to rush anybody. And you've got the shop too - which you have to exit through, naturally!
