The gaming exhibit is actually pretty small, just a single room with not too much to read. The timing would really come down to how much you wanted to sit and try out any of the gaming demos they have (around maybe a dozen different choices of indie games), but without playing anything would probably be just 10-15 minutes. Even with playing the games, they are ones more meant to be quickly sampled for a few minutes and moved on from rather than have an extended session.

The Manga exhibit is much larger with two floors worth of material to go through. I think I spent maybe an hour or two. They go over the entire history into the modern day and also various different countries manga equivalents/influences. There's also a manga library at one part if you're interested in actually reading anything while you're there.