How long does it take to build out a brood hive during good nectar flow?
Welcome to the group Lamb1. Is the queen new and mated? If so she will push the worker bees to draw out comb fast, a few weeks and likely it is almost full of comb. That being said, with strong nectar flow you have to be careful that the bees don’t plug all the available comb with honey and leave the queen little room to lay eggs. If you have a 10 frame brood box and 8 frames have comb I would suggest putting the super on. Even if you aren’t at that stage keep an eye on things every 3 days to make sure the comb you have isn’t full of honey— if you find that its time to super.
There are too many variables to put a number of days to your question Bill. I have had to add a super to a split in a few weeks when the is a good flow of nectar coming into the hive.
Do a check weekly and when there is 80% of the cells in the brood box have either brood, pollen or nectar in them it is time to put a super on the hive. By that time the bees should have built out comb on all the frames. Once you put the super on revert to your regular hive inspection time frame.