Declaring an emergency: How should you report fuel remaining?
There is no standard because there's no standard emergency. The response to "Say fuel onboard" (or the way the question is phrased) depends on who cares most about the answer.


Fuel Remaining (in minutes)
This is how you report it when you declare a minimum fuel emergency, and it's most useful for approach and tower controllers to know exactly how long they have to get you on the ground before your engine gets quiet (and if they can get you to an airport in that timeframe).
Fuel Remaining (in hours)
This is what you'll give for most general emergencies, and also what's entered on flight plans. It's the most generally useful number to a pilot or controller.
If you're VFR-only pilot stuck on top of an overcast and call ATC with a pan (urgency) message because can't see a way through the controller will want to know how far they can vector you to find a hole before it becomes a fuel emergency too. 
Fuel Onboard (in Pounds/Kilograms or Gallons/Liters)
The folks who care about this number drive around in ugly fire trucks wearing silver jumpsuits (the ARFF crews): If the worst happens they want to know how big a fire they might have to deal with, and how much foam they're going to need to handle it.