Depending on how big your crock pot is and how much sauerkraut you're fermenting it takes between 5 to 10 days.
If it's a smaller crock pot then it usually takes 5 days to ferment the sauerkraut in the crock pot.
When fermenting the sauerkraut in the crock pot lift the lid of the crock pot and check a few times during the first 24 hours to ensure that the cabbage has released enough juice to rise above the cabbage leaves by about an inch (so the sauerkraut is completely submerged).
Within 5 days the sauerkraut should be fully fermented if it's a small crock pot and for a larger crock pot it will take 10 days.
Homemade Sauerkraut is the best and I make a large crock pot of it which takes 10 days for the sauerkraut to ferment.