Whatever the length (not sure) I have had NO problems making an ADR there - even 50 days in advance. Then, just 37 days in advance I was able to change it to a diff day, and got a wide variety of times to choose from. It's like they were wide open! Not sure if this is the norm, or maybe the time we are going (Apr 26-May3) but just thought I'd throw it out there for ya!
Enjoy T-Rex!