A good way to switch between max and normal is to start with normal with a window down. About the time you feel it cool enough to roll the window up it is cool enough for to switch to max. Sometimes you may want to leave it on normal a little longer, but usually not by much.
The A/C is most effecient when it needs to cool the air the least. If it is cooler outside than inside, use normal. If it is cooler inside than outside, use max.
