There aren't any rules. It just depends on what stocks the shops and their suppliers have left and how long it takes to shift them. Typically you'll see bikes on sale at reduced prices any time from now until into September but some 2008 bikes will still be kicking around this time next year!
With the miserable summer I think there will be good bargains to be had. I've also heard from a few different places that next years bikes will be going up in price.