Our vet always had us wait 2 hours of no progress. When calves were pulled it took them a while to get up and going. We usually wait an hour. Basically if it looks like progress can be made and mama has energy we will let her try. Any signs of stress or improper positioning we put her in the chute and check her. If all is well as room is still available to deliver then we turn her out and let her try on her own.
We like the mama's to do it themselves.
Double R