No Consensus!
In my 22 years in the RN, during the days of JSP 318 and separate coloured pages, 'Endurance' was always defined as time to MLA (Minimum Landing Allowance) as stated by Spheroid.
Time till the engines went quiet was defined as 'Total Endurance'. IIRC this was a 'known' issue when RAF/Army helos were due to embark and a certain ammount of clarification used to go on.
Not sure what the situation is now that its JHC and JSP 500!!