How long should a cow be in labor before you call a vet?

It's best to not call the vet until you see the cow having trouble delivering her calf, and you're quite sure she's having trouble delivering on her own. There's a natural reason that the fetus isn't ready to be brought into the world at the due date, and it's not wise to interfere when it's not absolutely necessary to interfere, like when a cow isn't ready to calve on her due date. It's not uncommon for cows to go anywhere from two days to several weeks past the due date before they start calving.

The cow will let you know when she's ready to calve (as the fetus will let her know when it is ready to leave the womb). Showing of the water sac is a very obvious indication that the calf is ready to come out. If nothing is happening after a couple of hours or no further progress has been made, call the vet. Don't wait a day or more later, no matter if it's the weekend or in the middle of the night!