First thing is that it is not recommended to run your raw water cooled motor for more than 30 seconds without water at 1000 rpm. So, most people use a 15 sec test and multiply.
Second, what @ DW said about going to your pumps manufacturer and looking for specs.
Most of our pumps will put out around 6-8 gpm at 1000 rpm, and around 20-30 gal at 3500 up. I've seen recommendations by some of the race boat builders that for ultimate engine life and durability you need about 8-9 gpm for every 100hp produced but there's a lot of variables as water temperature etc..