The conditions that cause the coil to frost up the fastest, requiring the shortest defrost interval, are misty rain/very hi humidity with outdoor dry bulb temps in the thirties. Any time that the outdoor temp is below 40 the outdoor coil is going to be operating below freezing.
Cold air holds less moisture at any given relative humidity when compared to warmer air. A coil will frost up faster in 30 degree weather with hi humidity then it will at 10 degree weather with the same RH. In East Tennessee I set the timer to 60. 30 is a waste of energy in my area. I used to set the timer tom 90 and very rarely did it cause a problem. Setting at 60 in my area has never caused a problem for me. YMMV