You got me on this one so I took it to the Series 2 Club in the UK as one said it would depend on engine speed. However, here is the most complete reply:
I have never come across water pump flow rates or indeed any device that can be used by a garage to measure it, if we knew what it was.
The water pump works - or not... you'll soon know if it doesn't.
There is a visual check you can do... when the engine is warm and the thermostat open - be very careful in removing the rad cap - if you peer into the header top you will see a steady flow of water. If you raise the engine speed it will increase to the point where water could be thrown out of the rad top.
So you are looking for a steady flow of coolant.
Your thermostat should be working properly and of the right temperature range for your climate.
The rad hoses should be in good condition, slightly flexible and not brittle. All the clips in place and not chewing into the rubber.
The fan belt good and adjusted not too tight, not too slack.
The rad cap doesn't last for years, replace it every so often.
The radiator may/will silt/scale up and will need back flushing evey few years.
You should always run with a good quality antifreeze all the time - minimum of 33% up to 50% if conditions dictate.
(Only use water in an emergency get-you-home-situation.)
All this taken into consideration, you should have no problems with your cooling system...
Real trucks don't have spark plugs!