You can file N400 up to 90 days before the 3-year anniversary of your green card. (You must have been living in marital union with your U.S. citizen spouse for those 3 years).

but u remember i751 is one they approve first before moving to your n400. but don’t wait for i751 approval cause it won’t happen that first, go on and file n400 if u got what it takes, they will schedule both your 751 and n400 on same day n u will do two interviews at same time

I’m already a U.S. citizen. 😊

I didn’t wait for my I-751 to be approved and filed my N-400 as soon as I became eligible. During my interview, the immigration officer approved my pending I-751 immediately, and my husband didn’t need to be interviewed. The N-400 interview was conducted right away.