At a minimum they need to be separated for 8-12 weeks.
It really depends on the calf and cow.
I believe the age and health of the calf play into it. What you feed the calf and how much matters. Good healthy calves on good thick grass tend to wean pretty easy. Small sickly calves on dirt will remember how to nurse. Some calves never seem to wean.
Some cows dry off quickly. Some cows take longer. Once the cow is dried off completely, the calf isn't going to get much even if it does try to nurse again. The cow is probably more likely to kick at the calf if she is dried off. I try to rejoin mine a month or two before the next calf is expected so the cow is still dry and not about to start milking again.
When you put them back together, watch them really closely for an hour or two. They are going to sniff at each other, that is fine. If the first hour goes good, keep an eye on them for a week or so. You have to be ready to separate them again before the calf gets the cow back into milk.
I just weaned mine last week. I plan to try them back together around the middle of June. This is my 3rd or 4th year doing it this way and it has been working well.