Hi Mark,
 
I have always had best luck letting the hive re-queen itself. The bees will
know when their queen is no longer productive and start building queen
cells.
 
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> From: Mark J. Vogel <[log in to unmask]>
> To: [log in to unmask]
> Subject: The best time to replace queen?
> Date: Wednesday, August 20, 1997 2:13 PM
>
> I have two hives with queens that are two seasons old.
> They seem to be doing fine, but my understanding is the
> queens are past their prime at two years. My question is
> should I replace them now with new queens with the risk
> of losing them if we have a severe Winter, or should I
> wait until Spring to replace them?
>
> Mark Vogel
> Rockford, MN USA