I like to wait till after the flow is about finished, and then make splits, add new queens, requeen, etc.
I personally do not like to split my honey producing hives prior to the flow. The goal is to have the strongest hives making honey, and many times, people split right before the flow and screw that up. To many people use splitting as a swarm prevention measure, in my opinion, not the thing to do.
Splitting, requeening, and having brood breaks in the later half of June (for around here) means you had time to raise your own queens, buy better quality queens from elsewhere, accomplish this at a time just prior to the mites building up, and allows a brood break and a mite reduction (even treatment) before the fall brood cycle begins.
For northern beekeepers, we need to change our hive building mentality. Too many people go into winter with 10 hives, because they need 10 hives next spring. They lose half their hives, order packages with genetics they may not want, or buy early season queens that some say are perhaps poor quality (if even available), and split hives right before the flow many time lowering honey production, and it's a vicious cycle.
I like to use summer splits. If you need 10 hives for pollination or honey production, then split after the flow, build up to 15, and realize we are dealing with insects, cold winters, and mother nature will take a few from you. If you lose 5, then use the 10 that came out of winter as you planned, then build back up in the summer to 15. Stay ahead of the game. This allows you to make your own queens, split at a beneficial time, catch swarms to boost numbers, get better mated queens, and get off the "lose half and order packages every year" bandwagon.
and by doing so, you can split and wait whatever time you want for a brood break by doing this splitting and requeening. (who want a huge brood break in spring, wasting brood production time) And you will know that you have first year queens going into winter, and will reap the many benefits of young first year queens.