How early can we apply for Spouse Visa extension?

[thesamiad]

My wife is American and currently on the first 2.5 years of a Spouse Visa, due to expire Sep 2015. I have been trying to find guidance on when we can apply for the next 2.5 year extension and all I can find is that it has to be before the visa expires, and "towards the end" of the visa period. My question is whether anyone knows what is the earliest we could send our application in for the extension? My earnings will probably be higher towards the start of the year rather than the end, so it would be useful if we could include more of this period towards the financial requirement by getting the application in early.

Thanks in advance.

[MPH80]

Normally 28 days.