Hi. For hive defense/robbing prevention, it isn’t as much of a timeline as it is hive strength. With a month old nuc, keep the entrance small. It shouldn’t hinder a nuc to keep it reduced, on the other hand, it can easily save them from disaster. You can easily grow a nuc into a full grown hive through an entrance reducer. If you have one of the typical reducers with the very small opening (about 1”) that will rotate to a medium opening (about 2 or 3 inches), and the bees are having to take a number to go though the small opening, you can go ahead and change to the medium opening.
Just remember, there are a LOT of successful feral colonies out there, and most of them have a pretty small entrance.
Ryan