Do you mean gestation? Or the actual labor process?
Gestation is 111 days. The labor process takes a few hours depending on how many kits there are. If it's been an hour without any progress or the mom looks to be in distress, she needs to see a vet.
I was just reading it today on a piece of paper I had printed out info. on breeding chins a few years ago...I will try to find the sheet again and tell you the site...I was cleaning today, and don't know what I actually did with it. It has been major cleaning since yesterday. lol.