drauk,
It is different for different a/c and engines. Sometimes the carb heat doesn't open as far as it's meant to, or the cold air source for the engine is warmer than it should be.
A friend was recently involved in some work on the Lotus 340R, specifically about the location of the inlet air. It turned out to be in the bottom of the engine bay where the temperature got up to 40-50 deg C. When some decent trunking was installed and a slight ram effect created, the car was dyno'd and was found to be producing 10 BHP more than usual with better throttle response.
The point of this is, if a company like Lotus can get this wrong, what about 30 year old a/c!!
Without knowing the specifics it is hard to say whether the carb ht on your 182 is functioning correctly or not. I've never flown a 182 so I'm not au fait with it's handbook. Personally I like to see at least a 100rpm drop and a positive change in engine note and leave it for at least 10 elephants. This will ensure you don't just check the action, but also whether you actually have carb ice.
One question for everybody, where are you most likely to get carb ice forming? (I'm just interested to see the replies as I never get the correct answer from students!)