In Maryland, we advocate requeening in early September.   We still have about
1 1/2 month of brood rearing time left then, so the new queen has plenty of
time to get established.
 
We also advocate the "nuc" method of requeening (see Geroge Imirie's article
"Almost Foolproof Requeening" in the Brushy Mountain catalog).   With the
"nuc" method, you are running 2 queens at once for about 2 weeks, so you get
double the brood production.   And, if the new queen doesn't take, the old
queen will still be available to keep the colony going.
 
And finally, we advocate replacing two year old queens.   A queen that old is
almost certainly past her prime, and very likely to swarm next Spring.
 
W. G. Miller
Gaithersburg, MD