When am I eligible to file for naturalization?

The following copies from USCIS

If you are applying based on 5 years as a permanent resident or 3 years as a permanent resident married to a US citizen,
you may file for naturalization up to 90 days before you meet the "continuous residence" requirement.
For example, if you are applying based on 3 years of "continuous residence" as a permanent resident married to a US citizen,
you can apply any time after you have been a permanent resident in continuous residence for 3 years minus 90 days.
You may send your application before you have met the requirement for "continuous residence" only.
Therefore, you must still have been married to and living with your US citizen spouse for 3 years before you may file your application.
You must also meet all the other eligibility requirements when you file your application wiht INS.

For more details, please see
http://uscis.gov/graphics/services/natz/index.htm