Right or wrong, when in the cruise, I check the rpm and only pull the carb heat when the rpm drops or I notice the engine note change. Of course in the circuit and when reducing engine power etc the carb heat comes on regardless.
Could it be that you are anticipating carb ice and pulling the carb heat rather than reacting to known carb icing? usually it's only in really damp air where carb ice is a real problem and carb heat is necessary every few minutes.