When the Patent Office sends you an Office Action, you typically have a few deadlines for response. The OA will indicate the deadline on its front page: 3 months, but that time can be extended up to 6 months with fees.
If you respond before the 3-month anniversary of the mail date on the OA, you don’t have to pay any fees. If you respond by the 4-month anniversary, you have to pay a one-month extension fee. If you respond by the 5-month anniversary, you owe a more-expensive two-month extension fee. And if you respond by the 6-month anniversary, a most-expensive 3-month extension fee is due. The fees are all cut in half if you are legally considered a small entity. Your application is abandoned, retroactively, on the 3rd month if you fail to respond by the 6-month mark with the appropriate extension fee. You’ll then have to file a petition to revive and an additional petition fee.
The date on which you respond not only affects how much, if any, you pay in fees, but it can also affect the life of your patent when it issues. The term of a patent is 20 years from the filing date. However, because the Patent Office can sometimes take 2, 3, 4 years or more to process the application into a patent, the patent term is sometimes adjusted to make up for some of that delay. If you responded after the 3-month deadline to an OA, you do not get that delay back – so if you respond 3 months and 12 days after the Office Action was mailed, and your patent issues and is eligible for patent term adjustment, you will not get those 12 days back.