My pics drop to zero over salt by late September. Usage doesn't start again until late March.
My observations have been similar, although would not say a full "zero". The usage drops over 90% between late August and late September, and becomes mostly nocturnal.
Can't tell you how many times I've spent the opening day of archery season perched high over a salt lick, and never seen a deer. Never mind these same licks were being regularly visited during daylight by numerous deer throughout the summer and right up until a few days before archery season.