Those tanks should give you about 20 minutes run time with a oo tip in your cutting torch and a hour with a o in your welding torch @ 5 and 15 psi. Bigger tips and higher pressures will require bigger tanks They will not go flat with the tanks shut off under normal circumstances. Did you blow out the valves before fitting the regulators? Do the test agin using Ivory liquid soap @ 1 tablespoon soap to 1 pint water. Pay particular attention to the area around the valve stems and packings with the tanks closed then again with them fully opened and backseated. The quarter turn deal is no longer valid unless your tanks are over 20 years old.
You may want to have the local welding supply house check and refurbish the seats on the regulators if they have been damaged. You would be surprised at how fast tanks empty with just a small leak in that area because that is unregulated pressure.