If you are applying for permanent residence based on either a recent marriage to a U.S. citizen or through investment, you will be granted permanent residence on a conditional basis. This is for a period of two years. The expiry date of the conditional period is two years from the date of approval.
Form I-751 instructions :
If you are a conditional resident who obtained status through marriage, you need to file Form I-751 to remove the conditions on your residence. When the two year conditional period is over, the permanent residence expires and you can be deported and removed from the United States. In order to avoid deportation, you need to file Form I-751, Petition to Remove Conditions on Residence, 90 days or less before the conditional residence expires.
Once you submit Form I-751 to the USCIS, permanent residence will be extended for one year intervals till the request to remove conditions is approved or denied. If your application is based on marriage, birth certificates of children, certain joint financial statements, and letters from employers, friends and relatives are some of the evidence that applicant should submit.
Per Form I-751 instructions, if you are still married, the application should be filed jointly by you and your spouse through whom you obtained the conditional status. However, you can apply for a waiver of this joint filing if you entered the marriage in good faith and your spouse died subsequently. Also if you entered the marriage in good faith and the marriage was terminated because of divorce or annulment. Additionally, you can find in the I-751 instructions that you can apply for this waiver even if you entered the marriage in good faith and remain married, but have been battered or subjected to extreme cruelty by your U.S. citizen or permanent resident spouse or if the termination of your status and removal would lead to extreme hardship.
In case you are filing this petition jointly with your spouse, ensure that you file it during the 90 days immediately before the second anniversary of the date you received the conditional resident status. This is the date your conditional residence expires. You can file this petition at any time after you were granted conditional resident status and before you are removed. If you do not file this petition, you will lose your permanent resident status as on the second anniversary of the date when you were granted the conditional status. Then you can become removable from the United States.
If you did not file because of no fault of yours, you can apply later with a written explanation requesting for an excuse for filing late. You may be excused by the USCIS if you prove when you submit the application that the delay in filing was due to extraordinary circumstances beyond your control and that the length of the delay was reasonable.
Per Form I-751 instructions, you have to to include any conditional resident children applying with you to have their conditional basis removed, or they can file separately. You need to submit certain copies of documents indicating that the marriage through which you were granted conditional status was entered in good faith. So submit copies of as many documents as you can to justify this fact and to prove the circumstances of the relationship from the date of your marriage till the present date. The supporting documents, fees and the mailing address are detailed in the I-751 instructions.