Under some conditions you may need the carb heat ON all the time to prevent the ice forming ,the important thing is that even if you have half a suspicion that the RPM has dropped then use the carb heat and check the RPM after you go back to cold air if it is higher then you have had ice and more frequent checks are the order of the day.
30 sec is the very least i would use , a post above has pointed out the dangers of short appication of carb heat.