EC-155 has a RFM statement of 25 minutes following total oil loss. It has a main pump that uses the cooler, and a standby pump that bybasses it and only comes on line when the main pump has failed or ceased to draw oil. Should the standby pump fail to deliver pressure, that is considered an indication of total oil loss.
Below is a section of a 155 checklist I wrote some time ago, which mentions the 25 minutes, and my advice that it should ideally be used to allow for a safe ditching, rather than for continued flight.
SUBSEQUENT ACTIONS:
If single indication, land as soon as practicable.
If double indication, land IMMEDIATELY.
If immediate landing an untenable option, reduce speed to Vy, descend to low level and land as soon as possible within 25 minutes of first indication.
If noise, vibration or erratic engine indications occur, land IMMEDIATELY
CONSIDERATIONS:
The 25 minutes ‘run dry’ capability should not be intentionally used unless landing immediately really is impossible (e.g dense jungle) or, for instance, an offshore installation is close by. Ideally it should be used to enable a planned and controlled landing or ditching.