Straight off of the 4473 help for item 20c:
Sale of Firearms to Legal Aliens: A buyer who is not a citizen of the United
States must provide additional documentation (beyond a valid government-issued
photo identification that contains the buyer’s name, residence address, and date
of birth) to establish that he or she has resided in a State continuously for at least
90 days immediately prior to the date of the sale. (See Question 13.) Examples
of appropriate documents to establish State residency are utility bills from each of
the last 3 months immediately prior to the sale or a lease agreement that
demonstrates 90 days of residency immediately prior to the sale. A licensee may
attach a copy of the documentation to ATF Form 4473, rather than record the type
of documentation in question 20.c. Acceptable documentation to prove 90-day
continuous residency must be original documentation (e.g., utility bills, current
bank statements, rent receipts, mortgage payments, lease agreements, personal
property tax bills, documents issued by Federal, State, or local government
agencies, first-class mail issued by government agency, insurance policies, or bill
with current address or major credit card bill).

I'd ask them where it states 10 days...