Marcin: The split that raises the new queen is made up of all of the nurse bees (non-flying bees) of the hive coming out of winter, all of the brood (both capped and uncapped) and all of the stores. So it is very strong. The queenright portion of the split is made up of only foragers (flying bees) on bare foundation and is fed heavily. This is often called a "Fly Back Split." I shot a You Tube video of it.
Nectar begins trickling around the first week of April, but it does not turn on until mid-May and lasts almost the whole month of June. I get about 6 weeks. I always take a few days off around July 4 holiday and harvest and I don't bother putting the supers back on my hives after that. There is a little fall flow, but not enough for me to worry about.
I am on the Alabama/Florida line. My bees fly all year long, with only a few cold days here and there. Red Maple starts budding in late December and early January. If you don't split by March 15, the bees will split for you. It is a blessing and a curse. I get an early start, but so do the mites. If I don't get treatments in by the end of July, I start losing hives.