Last time I was trying to extend a MSP stay I think the rubric was six hours max (this worked for me, getting a 9am flight before a 250pm connection).

Also, as Fly Now goes, there were discrepant posts and threads on this, but the final word from NW Scoop was that if you are booked in T you do not need to have T seats available in order to FlyNow--as long as their is ANY availability in ANY coach bucket you can do fly now.