I will have to change Tom’s statement a little before I can agree with it. Carb icing “starts” to happen instantly in the form of a thin layer of frost. However, it can take some time before the affects get pronounced. And it is important to keep this part of the article in mind also: “Carb ice can form over a wide range of outside air temperatures and relative humidities. While the word “icing” typically brings to mind blustery winds and frigid conditions, carb ice can form when outside temperatures are as high as 100 degrees Fahrenheit with 50 percent relative humidity. At the other end of the spectrum, the risk doesn’t go away until the humidity falls below roughly 25 percent and/or the outside air temperature drops well below freezing.”
I have flown light airplanes for more than 40 years and owned a C-182 for a number of years. I have only experienced carb ice a few times as standard procedure is to apply carb heat in conditions where icing is likely to occur - generally during times of low power operation such as practicing approach to landing stalls and in making actual approaches to land. So, the idea is to try to prevent carb icing with carb heat rather than correct it once it has occurred. The reason being that by the time you notice power loss from carb ice, you may have accumulated a fair bit of ice. And when you apply carb heat, ice does what ice is wont to do when warmed and turns to water. Dumping a slug of water and ice chunks into your engine can make life interesting when in an airplane.
The worst carb ice encounter I had was with a 1975 VW Beetle than had been converted from FI back to carburetion. Yes, early mechanical FI systems were actually less reliable than carburetors. Unfortunately, I bought the car used after the conversion had been done and I was young then and didn’t think to look the engine over closely. It turns out that whoever did the conversion didn’t quite complete the conversion and had failed to install the hose that goes from the exhaust cuff to the air cleaner snorkel and provides warm air to the carb in cold weather and light throttle.
I was attending college in Erie, PA which is about 200 miles west of where I live. The first year I was there, I was heading home for Christmas break in early December and a wicked cold spell came through the day I was to leave. I left after my last finals, probably around 5 PM and I had a rider with me who I was dropping off in Salamanca, NY, which is about 80 miles into the journey.
The Bug started to run a little roughly as I came into Salamanca and I thought it was just some ice in the fuel lines so I added a can of drygas and continued on. The temp had been probably in the upper teens or low 20s when I left Erie, but was steadily falling and was in single digits in Salamanca well away from the moderating effect of Lake Erie. The Bug continued to run rough and lose power on the hills, but would gradually get better as I climbed the hill and then get really bad going down the other side. This continued for the next 70 miles getting progressively worse until the car completely died as I was climbing the hill just after passing Hornell, NY. Trying to get someone to stop and pick me up after dark, at temps around 0, is a whole nother story. And not really being well dressed for such weather didn’t help, but I digress.
The next day my dad and I went back to retrieve the Beetle. The first thing we did was remove the air cleaner and found a ring of ice at least 6 mm thick all around the inside of the carburetor. It was still near zero so the only melting that would have occurred was from the engine heat as it cooled down, so I suspect there may have been even more ice at the time the car died.
So, I would not call 70 miles and probably an hour and a half to be “instant” carb icing. The icing tends to form fastest and have the greatest affect when at part throttle as you have the most venturi effect cooling and the ice interrupts the airflow most then. Often engines will run OK at full throttle with some ice only to die when you pull back the throttle. I was lucky in that at OATs near zero and the typical low humidity of winter, it took a long time for enough ice to form to kill the engine.