I don't have a definite answer to this question but I would say the sooner the better especially if your wedding is surrounding a holiday weekend. Also keep in mind depending on the time of your wedding most hotel rates only are guaranteed for a certain amount of time for the following year. You can try calling now to see if your dates will be available with rates. A lot of hotel require a minimum of 10 rooms per booking. I was going to do this but I told FI I'm only checking for my OOT guest because he want his family to be in the same rooms. That is his business. I wish I had more of a better answer. I used to work in the hospitality industry but this is all I have as far as wedding.
When I went to check out hotels all of the managers told me to block out by November (my wedding is in June) so about 6-7 mo out? I think it also depends if you are getting married in a metropolitan city and if any community events will be going on during that time.