Maintenance fees must be paid to the USPTO on all issued patents, except for plant patents and design patents. A maintenance fee payment must be paid during the "window period" between 11 years and 11 1/2 years after the date of issue to maintain the patent between 12 years after the date of issue (grant) and the end of the patent term without the payment of a surcharge. The maintenance fee may be paid within a 6-month "grace period" after the "window period" if a surcharge is also paid. Patent Partner sets a milestone at the end of the "window period".
Law - Maintenance Fee Payment at 11 1/2 Years