How to Calculate Fuel Endurance for Item 19 in ATC Flight Plan CA48?

You could start with a complete sentence.

Fuel endurance is your operating time until empty. If you have four hours fuel on board, you have a four hour endurance. The CA48 endurance block is four digits, hours and minutes. 4 hours of fuel would be 0400. 6 hours, fifteen minutes fuel would be 0615, etc.

The purpose of fuel endurance is to determine how far that aircraft could fly given it's fuel state on departure, and is one of the principle data blocks when undertaking a search. If you had four hours fuel on departure and flew two hours before you disappeared, then you had two hours flying time at the last known position, which establishes the initial area of probability for a search. It's further refined by other flight details.

Endurance includes your fuel reserves. Accordingly, what you put on the form is also a legal statement.

Lots of various fuel burn rates with different phases of flight. Why measure with the micrometer when we later will be cutting with an ax? I'd rather not fly until fuel exhaustion anyway.

Back in the day we used a fuel burn for start, taxi, and take-off, deducted that. Then it was a cruise burn for the route, then fuel for an approach. If needed you went on to missed approach, then to an alternate. After that you wanted to have a min fuel for landing.

Endurance fuel(time) for an ATC flight plan is a ballpark figure.

I remember the Dispatcher practical. The Fed handed me a Boeing 727 AFM and had me generate a flight plan DFW-LAS. There are tables giving time, distance and fuel burn for climb and descent. Then you go to another chart and figure fuel burn for how loud you want to go during cruise.