3. Apply carb heat and observe engine rpm: If there is no drop, or a rise in rpm, with or without other indications, you have an iced engine. Leave carb heat on until rpm stabilises and condition 2. above returns.
thing. For sure it might take more than 30 secs to clear the ice, but having done so check 2 takes only a few seconds. Thereafter until out of suspected icing conditions do check 2 more frequently, perhaps every five minutes or so.
You might also consider changing altitude to get out of icing conditions, or leave carb heat on and accept the reduced engine performance & efficiency.
The point being that it takes only a couple of secs to check for carb ice formation, but much longer to clear the ice. So better not to let that ice form at all.