I am fairly sure it was just under 3 hours block to block (2hr 54 min rings a bell).
Aircraft involved was VR-HVY (now B-HVY) the 747-236F freighter CX purchased from BA at the end of April 1982.
The event was its C of A air test after all the checks and mods done by HAECO prior to entry into service with CX.
Time - May/June 1982.
A practice RTO at 100 knots was planned on the the first take off run possibly for simulator veracity checks. One of the inboard engines (RB211-524C2) suffered an internal FFR failure which resulted in it failing to decelerate when the RTO was initiated and it went into reverse while still running at take off power. Amazingly the reverser stayed attached but was seriously overstressed and there was a significant N1 overspeed. This occurred around midday. The test crew advised if they could get airborne by a certain time (say 1600) there was sufficient daylight remaining to allow completion of the test flight. The aircraft taxied back to the hangars and a double engine change team which was about 10-12 mechanics plus electricians set about replacing the engine. As per their policy CX had a nearly fully built up powerplant on hand. The aircraft redeparted from the hangar so no towing was involved other than pushing back from the front of the hangar to the taxiway. The test flight was completed the same day and HVY made its first CX revenue flight on schedule. Fortunately in those days a fully tested RB211 did not require any high power on wing running after installation so the engine run check was done in front of the hangar. The removed engine was converted to a D4 which meant the damaged thrust reverser and oversped fan and LP turbine were all discarded and replaced with new parts as part of the conversion.
HAECO quite often did 6 hour engine changes on the ramp including one with pax on board in the wee hours of the morning after an in flight return after midnight.
Cheers