Seats are released when NW's yeild management decides that there will be unsold seats on a flight. So really, there is not answer. If flights are selling well, why give away seats if you can sell them?. If there are open seats, why not give them away? I am sure there is a algorithim that determines when seats are released, but it is going to be impossible for anyone to figure out. You would also need to know many variables that only NW knows (unsold seats, which buckets, historical sales data, ...). Each flight will be different for each day. Flights can be booked 331 days out, but if historical data shows that flight is 100% sold in a high fare class, they are not going to give away seats until closer to departure, and only if it does not sell well. Elites can also get better availability, so the formula will change depending on your status with NW.

The standard answer here seems to be check often and try alternate routings, dates, airports, etc.

Good luck on finding what you want.