USCIS has issued the following press release regarding I751 receipts.
As of June 11, 2018, petitioners who file Form I-751, Petition to Remove Conditions on Residence, or Form I-829, Petition by Entrepreneur to Remove Conditions on Permanent Resident Status, will receive a Form I-797 receipt notice that can be presented with their Form I-551, Permanent Resident Card, as evidence of continued status for 18 months past the expiration date on their Permanent Resident Card. We are making the change from 12 to 18 months because current processing times for Form I-751 and Form I-829 have increased over the past year. Additionally, we will issue new Form I-797 receipt notices to eligible conditional permanent residents whose Form I-751 or I-829 was still pending as of June 11, 2018. Those Form I-797 receipt notices will also serve as evidence of continued status for 18 months past the expiration date on petitioner’s Permanent Resident Card.