What usually happens is that Apple
allows its stock of the old iMac to run down, just like what is happening with the AppleTV right now.
Once announced, the new item "usually" goes on sale immediately for around the same price as the old one.
Places like MacMall, CDW or Best Buy, who still have a stock of the old ones, will discount the older models.
As to when the new iMac will be announced.......That's anybody's guess