If an applicant decides to pursue an appeal before the Board of Patent Appeals and Interferences, an appeal brief must be filed. The appeal brief presents arguments and authorities on which the applicant (now called the appellant) will rely to convince the Board that each rejected claim should not have been rejected. The appeal brief must be filed in triplicate within 2 months from the date that the notice of appeal is received by the USPTO or within the time allowed for response to the action appealed from, whichever is later. Either of these time periods may be extended by up to 5 months by payment of extension fees, however, in no event can the appeal brief be filed later than 6 months after the mailing date of the non-final Office action or later than 7 months after the date the notice of appeal is received by the USPTO, whichever is later.
The position of the USPTO on this issue is described in the following section(s) of the Manual of Patent Examining Procedure (MPEP):
MPEP 1206 - Appeal Brief