This is a good report to read concerning carb icing:-
The investigation did not find any evidence of a failure within the engine but the atmospheric conditions were conducive to carburettor icing.
Each of the local training establishments that I have knowledge of, all say that there are three times when you should apply carb heat... 1. If you think carb icing is likely. 2. Any time the engine is run below 2000rpm. and 3. As the C in the downwind pre-landing BUMFFPITCHH checks.
If you conduct a straight in/direct approach you invariably won't fly downwind! For this reason when teaching I always refer to them as (Pre) Landing Checks. As the report above shows the practice of selecting hot air in the pre landing checks won't necessarily give you the protection you need. You need to select hot before reducing power for the approach and leave it there until landing and/or going around.