I think it is important to note that the first indication of carb ice in cruise flight should be the pilot noticing an uncommanded reduction in RPM (or MP in an aircraft with a constant speed prop). At that point the first action should be to suspect carb ice and apply full carb heat.
There is nothing wrong with the practice is to check for carb ice by applying carb heat at some regular interval, but if you were to find carb ice during one of these "checks", I think one should be asking themselves why they did not notice any earlier signs of ice developing.
Finally the RPM drop when carb heat is initially applied has nothing to do with carb ice it is simply the fact that the hot air has enrichened the mixture to such an extent that the engine will lose some power due to a overly rich mixture. This drop only proves the carb heat control is actually working. Only if there is a rise in RPM (MP) after the heat has been applied can we deduce that there was carb ice and it has now cleared.
How fast carb heat will take effect depends on the efficiency of the carb heat installation. Some airplanes will deliver massive amounts of instant heat others are less effective. Personally I would say that a few second application of heat may not be effective and the fact the RPM returned to the same value as before the heat was briefly applied may only be due to the fact that the resumption of cold air has returned the mixture to a normal value and therefore allowed the engine power to pick up, but without clearing ice that was still there.
I teach my students that if they apply carb heat they should leave it on for at least 10 seconds. In every aircraft I have flown this would be sufficient to provide unequivocal evidence of icing.
Finally carb ice is most likely to develop at low power settings so it is most dangerous in the landing and takeoff phases and therefore extra attention should be used in those phases of flight.
A related item IMO is importance of knowing the static RPM (fixed pitched prop) or field baro MP (constant speed prop). Sitting with the engine idling while you are waiting to takeoff is a prime time for carb ice to develop and I often give the engine a shot of heat as I manoever into position for takeoff on days conducive to icing, But your last line of defense is to assure that the engine is making full power is to check that you are getting static RPM as soon as you have applied full throttle.