Hi Smurfette,
Under 'old rules' the qualifying period for ILR is actually 24 months living in UK as Spouse (this does not include any time under Fiancee Visa)
The earliest you can apply for ILR is 28 days before reaching the end of this 24 month qualifying period. Which may or may not coincide with your Spouse Visa expiry.
That's the earliest you can apply.
The latest you can apply is at current visa expiry.
If you intend to apply at PEO you can apply anytime between December 23 2013 - January 17 2104
If you want confirmation of eligibility outside of these dates you'll need written agreement from UKBA (has been done before under similar situations)