I think it varies by property and type of event. I know the local properties have a lot of control in these cases.

I've booked groups of 25-50 rooms where unused parts of the "block" were returned to general availability 7 days before the event. I've also been on a committee for a large event where we were able to confirm the final guest list 48 hours prior to the stay. (This was one where we probably filled up over half of the hotel.)

So the only real advice I have is: the local group sales office at the property might have useful data for you, but the website or 1-800 number probably will not.