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.
Sauté, in its original french term, means cooking in a pan on super high temperature. Sautéing onions takes no more than 3 minutes. If it takes 10 minutes, it's not sautéing as your oven is definitely not at the maximum setting or close to it.
http://www.seriouseats.com/2011/01/the-food-lab-real-french-onion-dip-homemade-super-bowl-recipe.html
http://www.seriouseats.com/2011/01/the-food-lab-real-french-onion-dip-homemade-super-bowl-recipe.html
He goes into great detail about the chemistry involved, including the Malliard reaction, which occurs more quickly in alkaline conditions. By adding a little baking soda, a person can shorten the time from 45 minutes to 15-20 minutes, according to him.
I find that a lot of people confuse sauteing with sweating, which could be where somebody would get the idea that it would take 10 minutes to saute onions.