So if 5 minutes is an hour, then 5 seconds is 60 seconds. How long does it take for the van to drop into water? A few seconds, so that is 60 seconds. So why is that going on for a long time in the dream?
don't forget, the chemist who designed the "super-sleeper" concoction explained how the timing was completely different... you're going by the timing that Leo explained to Ellen when she had her first dream training... this timing scheme is different. The extremely potent tranquilizer made things MUCH longer... go back and watch the part where the whole team is in the warehouse discussing this (wherein they discuss how the 3rd level down will be 10 years)