Based on the information and timescales you've provided, the next step would be ILR (Indefinite Leave to Remain)
Under the 'old rules' (pre July 2012) the qualifying period for ILR is 24 months living in UK as Spouse
(this does not include any time under Fiancee Visa)
Application can be made 28 days before reaching the end of the 24 qualifying period.
Before applying for ILR she will need to have a pass certificate for "Life in UK" test.
She will be eligible for Citizenship after 3 years living in UK as spouse provided she is free from immigration time restrictions on the day she makes the application. Also providing she was actually present in UK 3 years to the date.
(check the requirements - refusals will be made on this point)
By the way time spent in UK under Fiancee visa status can be counted towards Citizenship.
I'd suggest a review of the detail on the UKBA website Requirements for naturalisation if you are married to or the civil partner of a British citizen
Looks like it's going to be an expensive time if you stick with the allowable timetable
Does she already have a pass certificate for Life in The UK ?