The prosecution process can conclude in one of two ways: abandonment of the application or issuance of a patent upon payment of an issue fee. The applicant is notified by the Examiner that a patent will be issued in a document called a notice of allowance. The issue fee must be paid within 3 months of the date of mailing of the notice of allowance. This time period may not be extended by petition and payment of a fee under 37 CFR 136(a).
The position of the USPTO on this issue is described in the following section(s) of the Manual of Patent Examining Procedure (MPEP):
MPEP 1306 - Issue Fee
Law - Issue Fee Payment