In which cases can the “Petition to Accept Unintentionally Delayed Payment of Maintenance Fee in an Expired Patent” form be used?

This form can be used to reinstate a patent when the petitioner submits it (along with appropriate fees) within twenty-four (24) months after the six (6) month grace period provided in 37 CFR 1.362(e). The form cannot be used to pay successive missed payments, e.g. if the applicant did not pay the 4-year and the 8-year maintenance fee.

USPTO FAQ - Questions and Answers (2007JUN01)