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 3 years and 3 1/2 years after the date of issue to maintain the patent between 4 and 8 years after the date of issue (grant) 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 3 1/2 Years