When to replace a Queen

I haven't reached the point of needing to replace a Queen because of her age yet but this part always puzzles me. My first Queen hatched in June 2009 and the bees superseded her the following May at which point I managed to make gain another colony from it too.
Some state you shouldn't have a Queen more than 2 years old, and others say it is in the August after her 2nd full season that you replace her.
So I have 2 Queens raised in May '10, provided that they perform well would I replace them in August 2012 or should it be earlier/later.
I've read some people advise to requeen in March using a Queen from a nuc brought through the winter, however they're based in England- would that be a benefit in Scotland?

Young queens are the best, but they are generally fine for two seasons, sometimes more. I've seen 5 year olds. Your bees are moderately pure Amm if I remember correctly and they are generally longer-lived than other types and with a greater tendency to supercede. Also, the more moderate forage in Scotland means that the queens don't age as quickly as in highly productive places. With luck your bees will again supercede your older queen and do the job for you.

Is the best scenario to supercede the old queen? If the queen is not marked, is there any evidence that she has been superceded?

I think 'if it ain't broke, don't fix' it applies to queens. Some get superseded within a month of starting to lay suggesting that they have not mated properly or have some other problem such as nosema.
Others just go on and on like Maggie Thatcher once suggested.
I would change a queen on the grounds of aggression or having offspring obviously hybridized but not on the grounds of age.
If she starts to run out of sperm in her spermatheca the bees will supersede her naturally.
If you don't mark queens it's quite possible that a supersedure could take place unnoticed, especially in late summer or autumn when weekly swarm control inspections are not needed.

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Originally Posted by gavin

... With luck your bees will again supercede your older queen and do the job for you.

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I like the sound of that, I have more faith in my bees knowing what needs to be done than me. So for a 'if it ain't broke don't fix it, but have a backup just in case' I'll just try and have a couple of nucs in hand over next winter?:)