Hard to say without knowing how much lots is, but the main thing is even heat (i.e., use a heavy bottomed pan) - and don't crowd it. If crowded, they'll stew in their juices and take much longer to caramelize.
be sure to use a pot with a heavy bottom, like a clad braising pan....as the juices cook out of the onions the sugars will begin to caramelize and you'll want some nice slow even heat so they don't burn at that point. Keep stirring!
The Kitchn offers a caramelized onion recipe in the slow cooker if you're interested:
http://www.thekitchn.com/how-to-caramelize-onions-in-the-slow-cooker-cooking-lessons-from-the-kitchn-193413
I love to use a large roasting pan in the oven if I'm doing a large batch. Just bake on a low heat (275-300) for hours on end, stirring occasionally, until dark brown and delicious!
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