After mowing my pastures every day, I blow off the tractor, and the screen radiator protector. After pasture mowing has all been completed, I'll then wash it off with soapy water and a sponge, and then hose it off.
During the rest of the year, I'll blow it off after every dusty/nasty use, such as spreading road rock on my place, and then wash it completely when I'm done.
In the winter and spring when it's muddy, and I'm using it only to basically put round bales out for the cows, there's usually mud I have to go through each time to get to and from the hay bales. I'll wash it about once a month then.
It must work since I just sold my Kubota I had for 6 years and it looked almost like new.