The best way to do this would be to take a stopwatch, and whenever you are actually fishing (ie plundging the harpoon etc) the clock is running, and when you hit full inventory, or a random event, or a spot move, you stop the clock until you continue fishing. and you time how long it takes to get X (500 should be good, so we can get a good sample, that's about 20 loads) amount of monks.