The float pool carries a much higher salt concentration than the other pools — enough that guests float with almost no effort, which is the entire point of the ritual.
Because the water is kept cooler than the main pools, thirty minutes is close to the practical limit before guests start noticing the temperature rather than the float itself.
Earplugs are offered at the entrance; most guests float with their ears underwater, and the change in what you can hear is, for a lot of people, half of what makes this ritual different from the others.