Copied from USCIS:
Starting Sept. 4, 2021, USCIS is extending the time that receipt notices can be used to show evidence of status from 18 months to 24 months for petitioners who properly file Form I-751, Petition to Remove Conditions on Residence, or Form I-829, Petition by Investor to Remove Conditions on Permanent Resident Status. We are making the change from 18 to 24 months to accommodate current processing times for Form I-751 and Form I-829, which have increased over the past year.
Conditional permanent residents who properly file Form I-751 or Form I-829 will receive a receipt notice that can be presented with their Form I-551, Permanent Resident Card (also known as a Green Card), as evidence of continued status for up to 24 months past the expiration date on their Green Card, while their case remains pending with USCIS.
Additionally, we will issue new receipt notices to eligible conditional permanent residents who properly filed their Form I-751 or Form I-829 before Sept. 4 and whose cases are still pending. Those receipt notices will also serve as evidence of continued status for 24 months past the expiration date on their Green Card.
As a reminder, conditional permanent residents who plan to be outside of the United States for a year or more should apply for a reentry permit by filing Form I-131, Application for Travel Document, before leaving the country. For more information on International Travel as a Permanent Resident, see our Green Card page.