how much time after weaning off reintroducing calves into the herd?

We've been doing the system of keeping cow and calf together for a week, than seperating at night for 3 months, than completely separating.
Now we are thinking about when we can reintroduce them to the herd. They are weaned off 2,5months ago. I understand the answer will be 'if they arent drinking anymore', but i'd like to not try that with trial and error, because separating them again will give nightly concerts again, i'm afraid...