Actually plumbers use high compressed air for checking leak when they
finish a project. Clos all the valves, faucets, then pressurize the
eintire run, then leave it couple days. If there is pressure drop,
they have to find the leak before they can commission the system.
Real SMALL leak can take a day to produce a drop of water.

Actually what we do is fill the system with water then put a Hydrostatic
pump on it pump it up to 125 PSI. It is a little more work to fix a leak put
you find them faster. Plus it is much safer. Some systems do require air
only to test them. You only about 15 psi to find leaks, with soapy water.