We have moved them up by breeding on their calving heat (less then 3 weeks post calving), in theory the uterus has returned to condition and tone etc. But the following year they were slower having their first heat. That's strictly anecdotal because we only did it one year and only a couple of cows.
Sometimes a heifer will take a litttle longer to come back in after haveing their first calf. They can come in as earlyt as 3 weeks and should come back in by 60 days or they need to go. We try to give our heifer 1 heat cycle before we breed them back.