A gallon over a big enough area, with a roof that's dark, in 90+degree ambient temperature and direct sun... evaporates within a few minutes. Sometimes < 1 minute by visual inspection, anyway.
re: "doing any good", yes, that's hard to say. The ROI may not be for years, if ever. It was as much a "I have an idea,let's see it through" project as a real money saver.
The attic is insulated, and with a radiant barrier, to boot.