I dunno what does it one way or the other, but I empty mine every 1000 miles or so (500 miles sometimes when I drive to and from S. Tx) and sometimes it has maybe a 1/4"-1/2" in it, and sometimes it has 1 1/2" in it. So I am assuming my driving style within that 1000 miles plays a huge part on that. Who knows? Temps may do it, Humidity may do it, who knows. But if I were you I would start off checking it often, then you can always slow down on the frequency at which you check it if you don't find much each time. I have waited more than 1000 miles, maybe 1500 or more one time and it was......3/4 full!!!!! It may have been 2000 or more miles I don't know.