It's great to hear that your calf is now nursing from its mother! Generally, it’s best to allow mother and calf to stay together as soon as the calf is able to nurse properly. Being with the mother provides not just nutrition but also important bonding and socialization.
If you need to separate them, try to keep the separation to a minimum, ideally no more than a few hours at a time, especially in the early days after birth. Long or frequent separations can stress the calf and the mother, and may also disrupt the bonding process.
The calf should naturally go back to its mother if they are reunited, especially since it is already nursing. Make sure to monitor their interactions initially to ensure that both are comfortable and that the mother is receptive to the calf nursing again.
If there are ongoing issues or concerns, consider consulting a veterinarian or an animal husbandry expert for guidance specific to your situation.