I've heard within 6 mos is acceptable, but it seems that most folks want within 4 weeks.
That being said, both my 66 Chevelle and my 69 Camaro have 10 bolt, non posi rear ends (3.36:1 and 2.73:1 respectively) that were built in mid October of the prior year - several months ahead of final assembly of either car.
The Chevelle was my grandparents' car and has been in the family since they purchased it on March 30,1966. It went through final assembly in Flint, MI around March 25, 1966. I have the original paperwork and the POP. According to the axle code, the stamped numbers on the OD of the ring gear, and the POP, the rear axle was assembled on October 27, 1965 at the Buffalo Gear and Axle Plant - 5 months ahead of final car assembly.
My 69 Camaro went through final assembly in Norwood around the 19th of January 1969. The rear axle in it was assembled on October 18, 1968 at the Detroit Gear and Axle Plant - 3 months ahead of final car assembly.
As always, there are some exceptions to what is usually considered normal.
Also, if you are interested, I believe I have a distributor, date coded mid Nov 1968, for 327/210 hp, but for manual transmission. I don't have any reference numbers with me, but seems like it is only the last didgit that is different in the part number stamped on the body between auto and manual trans. Reset the centrifugal and vacuum advance and you'd be ready to go.