On the other hand, this gave me a really dark dulce de leche and the stirring time was greatly reduced since most of the evaporating was done in the crockpot instead of on the stove.
However ... and this is a big however. The timing on this is going to be entirely dependent on how hot your crockpot gets.
I've heard some people say that the low setting is hotter than it used to be when crockpots first came out. Maybe. But maybe not. My newest crockpot seems a little slower than my older one, so maybe it's more about brand or model.
But that's okay. The slow cooking meant that there wasn't much risk of scorching the milk.
If your crockpot runs hot, you might need to do some stirring now and then, and for sure you'll need to watch it towards the end of cooking and stir, as needed.
Crockpot Dulce de Leche
2 cups sugar
1 teaspoon baking soda
1/2 teaspoons salt
1 tablespoon vanilla
Put all the ingredients in the crockpot and crank it up to high. Stir, as needed, to melt the sugar, until the mixture starts to foam up spectacularly. It might grow to about double its size if you let it. Turn the heat down (or off) until the foaming subsides.
Set the crockpot to low heat, and let it cook, uncovered, until it has reduced to about 1/4 the original volume, and it is as thick and syrupy as you want it. That's right - 1/2 gallon of milk should reduce to about 1 pint.
Please note that the crockpot needs to be uncovered, or you will be cooking until doomsday. You want the liquid to evaporate. If you're worried about small flying objects landing in the crockpot (like if you're making this in the summer and your pet fly is buzzing around) a splatter screen over the top of the crockpot will let steam escape while barring access to pesky pests.
After 8 hours in my crockpot the mixture hadn't reduced enough, so I decided to hurry the process along. I transferred the mixture to a pot and put it on the stove and let it cook at a bare simmer while I stirred it (my crockpot didn't break a simmer at all, except on high, which was way too high for long cooking).
It didn't take much longer on the stove before I had a thick sauce, but I didn't cook it all the way down to 1/4 of the original volume. I ended up with about 3 cups (1 1/2 pints) of a sauce that is pourable at room temperature. You can make yours as thick as you like, all the way down to something that's almost chewy.
Depending on how hot your crockpot gets, you might (probably) be done in a shorter time. The low setting on mine is very low. You might also need to stir the mixture to keep it from scorching, if the pot has hot spots or if it cooks hotter.
Towards the end of cooking, you might want to raise the heat to high and stir continuously to finish it. Once it gets to a certain thickness, it seems to progress much faster, so at that point you might want to be there watching it.
Now that I know how long this takes in my crockpot, I'll probably start the process at night and be ready to watch it during the day. Starting it at noon probably wasn't the best idea I've ever had. However, with a different crockpot, it might take only 4 or 6 hours. Until you've tested this, I'd suggest starting it early on a day when you can be around to watch it, and see how long it will take. The next time, you'll know what to expect.
Of course, if it's not done, there's no harm in taking it out of the crockpot and refrigerating it, then continuing the next day.