There's no requirement to apply for N-400 ever. But if eligible and desired, you can apply for it.
If you were LPR for over 5 years, you can apply under 5 year rule. It's easier and doesn't require as much evidence or proving marriage. If you're between 3 and 5 years of being LPR, you can, either wait for eligibility under 5 year rule or apply under 3 year rule. In each case you can apply 90 days before anniversary. All other requirements (physical presence, continuous residence, good moral character) apply any time you apply for citizenship.