We're set for a mid-November trip. When I called to make the flight reservations (with Northwest), they said I could get seat assignments 90 days out - which I did, BTW.
Also, NWA's policy on seat assignments was interesting, and new to me. I couldn't request bulkhead (front-row coach) seats in advance unless I was a "special assistance" passenger, and couldn't request exit-row seating in advance at all.
However, on the day of the flight, I could call to request bulkhead if it wasn't already taken, and can ask for exit-row only at the gate.
HTH.