Hi T L,
Your spousal application is granted for a period of two years of which I'am sure you are aware of. Now to preserve your stay in the UK following the expiration of this leave you are advised to make an application for ILR which is decided on a discretionary basis by the Home Office. However usually applicants will be granted up to a period of 2-3 years leave. Thereon you could lodge an application for British citizenship should you wish.
Therefore to be on the safe side you are advised to lodge the ILR application up to a maximum of 1 month in advance prior to the expiration of your leave as a spouse (March 2010). Please ensure you retain a copy of this application to ensure future reference.
Hope that helps
Regards.