Informal drawings can be submitted when a patent application is filed. Even if formal drawings are submitted at that time, they can contain formatting errors called "informalities". Correction of drawing informalities is affected by filing new formal drawings. The filing of new formal drawings must be within 3 months of the applicant's date of receipt of the notice of allowability (PTO-37) from the USPTO. This period of response 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 notice allowability.
The position of the USPTO on this issue is described in the following section(s) of the Manual of Patent Examining Procedure (MPEP):
MPEP 608.02 - Drawing
Law - Correction of Drawing Informalities