If my cylinder of air lasts 60 minutes while I am at the surface breathing normally, assuming all else is the same, how long will it last at 20 metres/66 feet breathing normally? 60 minutes

Let the volume of air be V. at atmospheric pressure, that is 10⁵ Pa

At 20 m below surface pressure will be

atmospheric pressure + hdg

10⁵ + 20 x 9.8 x 1000 = 2.96 x 10⁵Pa

At this pressure volume V becomes V / 2.96

This volume will last 1/2.96 times time that is 60/2.96 = 20.27 minutes.