The mask check is 5 seconds at 100%,
Company policy? I recall our airline had a policy of making sure the oxygen level actually dropped during the test (747-400). It certainly kept our engineers in a job, topping up crew O2. You may find that crews don't time the 5 seconds, so tests may run longer.
Was the last pressure check taken after the tank temperatures had stabilised on the ground. Some tanks are close to the fuselage skin, so skin temperatures can affect the readings (circa 100 psi). Note that some Airbus types have temperature compensation.
Anyway, if in doubt.... The plumbing from the 777 MEC to the cockpit is probably easier to access than many aircraft, so leak checks are probably not too onerous.