Here, telling the truth about how to prepare onions for French onion soup, is Julia Child: "[C]ook slowly until tender and translucent, about 10 minutes. Blend in the salt and sugar, raise heat to moderately high, and let the onions brown, stirring frequently until they are a dark walnut color, 25 to 30 minutes." Ten minutes plus 25 to 30 minutes equals 35 to 40 minutes. That is how long it takes to caramelize onions.
But if you take the time to do it right, you'll be rewarded with some mighty delicious cooked onions. The article at Slate looks into how chefs come up with their timetables, which are different from what goes on in your kitchen. Link -via Metafilter, where you can find a lengthy but helpful discussion on onions.
(Image credit: Flickr user Esteban Cavrico)
*Actually less than ten minutes, but the linked article is only about caramelization.