G'day Gaz. I would say indefinitely, though most here age their water in the way you asked about for 24hrs to achieve a stable pH. The aerating allows gases (often CO2) from being pressurized in the process of delivering city water to you home to be released, causing some measure of pH swing to occur from when it first comes out of the faucet. Once the water is aged, it becomes stable and you can store & use it at your leisure.