This article is from the FAQ, by with numerous contributions by others.
Five years from the date of entry as a lawful permanent resident.
If married to and living with citizen spouse in marital union for
at least 3 years before filing the application, the residence period
is shortened to 3 years. You can file the application 3 months
before the residence requirement is met. Also, you have to be a
resident for 3 months in the state or INS district where you
are filing the application.