Instruction within the I797 - "The above application has been approved. Prior to receiving your permanemt resident card you may be required to report for biometrics processing (photo / fingerprint / signature). Please do not take any action at this time. If you are required to report for this processing, you will receive another notice advising you of the date, time and location to appear.
If you have not received your pernament resident card or the above metioned notice to appear for biometrics processing within 90 days, please call this office at the number listed below."