If you are a spouse of a US citizen, you may be eligible for citizenship. To qualify, you should have been a permanent resident (green card holder) for at least three years. You should have been living with the same US citizen spouse during such time. In some instances, spouses of US citizens employed overseas can qualify for naturalization regardless of their time as green card holders, per section 319(b) of the Immigration Nationality Act (INA).
Other normal requirements are that you should be 18 yeas or older and a permanent resident for at least three years immediately preceding the date of filing the citizenship form, N-400. You should be in marriage with your US citizen spouse, who has been a US citizen during all of such period (during the three years immediately preceding the date of filing the application and up until examination on the application. You must have lived within the same state, or USCIS district with jurisdiction over your place of residence for at least three months before the date of filing the application for citizenship. You should have maintained continuous residence in the US as a green card holder for a minimum of three years immediately preceding the date of filing the naturalization application.
In addition, you should have been physically present in the US for a minimum if eighteen months out of the three years immediately preceding the date of filing the form. Also remember, you should be able to read, write, and speak basic English and have knowledge of US history and government (Civics). Another important requirement is that you should have good moral character, attached to the principles of the Constitution of the US.
If you are employed abroad :
If you are employed by the US government, ( including the military or other qualifying employer ) and stationed abroad in such employment for at least a year, you can qualify for naturalization, per section 319(b) of the INA. You must be present in the US pursuant to a lawful admission for permanent residence at the time of examination on the naturalization application and during the time of naturalization, and fulfill all the requirements listed above. However, you can be exempted from certain requirements .
There is no specific period as a green card holder, but you have to be a green card holder. Additionally, no specific period of continuous residence or physical presence in the US is mandatory. Other than these, you need not be in marriage with the US citizen spouse for a specific period. However, make note that both, you and your US citizen spouse have to be in marriage.