Thumping will work year round except when the fish are already shallow like in the spring time. Sometimes it works then also.
From what I have found if the fish are less than 12 to 15 ft. deep it will not attract them as well as other times.
Late fall and winter are ideal times for thumping. Summer, when the fish are deeper trying to find cooler water, it works fairly well also.
I look for scattered bait balls and maybe a fish or two in water that is deeper than 15 ft. on my sonar, and start thumping.
Usually the bait will move in first, then you will see diagonal lines from bottom left to top right on your sonar. That is fish chasing shad up from the bottom. Hold on to your rods tight.