Going by Japanese release dates since those are probably the best way to look at this:
Yellow was released two and a half years after Red and Blue.
Crystal was released one year after Gold and Silver.
Emerald was released two years after Ruby and Sapphire.
Platinum was released two years after Diamond and Pearl.
Thus, while there is nothing saying that this pattern will hold, if you want to look at this by examining past trends, history would say the release date would be in mid-September of 2012. However, just to be clear, any predictions like that are just guesses, and there really is no way to know exactly when to expect a next game to be released, assuming there is one.