Well- since everyone is different it "happens" differently. Usually you should cycle through the entire thing in 2 hours. The uh, explosion can happen once, or twice or you name it. There are times when you think you're "done" and then you get a surprise. The best thing to do is to start the swf on the weekend to get an idea of what to expect.
On another note: some people aren't affected with explosions and you might be one of them!