From a practical perspective what is the current requirement and how is that demonstrated? i.e. is it a one time demo under a set of conditions set or driven by the manufacturer or is it a standardised format from the regulator?
This is an excellent question.
From FAA FAR 29.927:
"(c) Lubrication system failure. For lubrication systems required for proper operation of rotor drive systems, the following apply:(1) Category A. Unless such failures are extremely remote, it must be shown by test that any failure which results in loss of lubricant in any normal use lubrication system will not prevent continued safe operation, although not necessarily without damage, at a torque and rotational speed prescribed by the applicant for continued flight, for at least 30 minutes after perception by the flightcrew of the lubrication system failure or loss of lubricant.
(2) Category B. The requirements of Category A apply except that the rotor drive system need only be capable of operating under autorotative conditions for at least 15 minutes.
(f) Each test prescribed by this section must be conducted without intervening disassembly and, except for the lubrication system failure test required by paragraph (c) of this section, each part tested must be in a serviceable condition at the conclusion of the test."
The FAA 30 min. loss-of-lube qualification only requires a single test article passing a single test procedure. On one hand, it is quite difficult to design a high-performance helicopter MRGB capable of meeting this test requirement, and conducting the test itself is quite expensive. But on the other hand, due to the large number of variables that can potentially have a significant impact on the results of this particular test, a single test article passing a single test does not really provide a statistically relevant validation of the system's design and reliability analysis. As my high school chemistry teacher was fond of saying about experiments or testing,
"Nothing actually happens unless it happens more than once."
In the article linked, AgustaWestland's chief gearbox engineer (Giuseppe Gasparini) claims there are basically five factors that determine an MRGB's loss-of-lube operational capability. But in reality there is another factor he did not mention which has more potential impact than all the others combined. That is the design trade-off between MRGB weight and reliability. It is theoretically possible to design a rotorcraft MRGB that is capable of operating for hundreds-of-hours with nothing more than a mist of oil for the gears and bearings. But such an MRGB would likely be many times the size and weight of conventional designs.