I so understand what you are going through. We were married on January 19th and I have to say that the room block can be the biggest stress factor!
Now, between my wedding party and all of my DVC reservations, we were in good shape before others even started booking...but this drove me nuts.
I told my guest in June to book their rooms, that if they had problems to call, etc. 5 weeks before the wedding, several people who swore they were coming still hadn't booked. I had to call/email everyone to let them know that the block would be going away 30 days before the wedding. It was not fun. that being said, most people booked after I reminded them that the room block ended in a week. HOWEVER, two of my cousins did not book and they ended up getting SCREWED! My one cousin ended up off site at a super-sleazy hotel and the other ended up paying BIG bucks to stay in the only place that had any rooms available!
So my advice...I would send out an email newsletter, a paper newsletter, or just call. Maybe phrase it that you are worried that there will be enough rooms and to book early to make sure they get the category (value, moderate, deluxe) that they were hoping for.
Also, if you get close, I have seen brides post that they have rooms available in their block and a lot of DISers have jumped at the opportunity to get the discounted price. Just an idea though...
To answer your question though, most people booked about 2-3 months prior to the wedding...
Good luck!