I once had a cow lose a month old calf to meningitis. We really fought to save that one, but eventually had to put him down. I had a month old twin calf that was rejected by the cow so we decided to try to graft. We put her on every day for a week, but each time we left the cow lose, she would push the calf away. I was ready to give up when I talked to a fellow with a large registered operation. He told me it can almost always be done, but his longest took almost a full month of putting the cow in the head-gate. I said that sounded like too much work and he said, would you rather feed it twice a day on the cow for a month or feed it twice a day on a bottle for 4 months. He also pointed out that milk replacer is expensive and that once the calf is grafted, it can eat whenever it wants and will be a bigger healthier calf at weaning.
We decided to keep on trying, and at about 2 weeks we went down to put the cow in the headgate, and the calf was nursing on her own. After a few more days of confinement, just to be sure, we put them back in the herd. She loved that calf like her own from then on.
We have since successfully grafted many other calves, and they are all different. It depends on the personality of the cow, the age of the lost calf, and the aggressiveness of the calf being grafted. Most only take a few days, but many will take close to a week. The important thing is to not give up. I do put "Orphan no more" on now, and I think it speeds it up, but even when I did not use that, I have always eventually been successful.