I can almost guaruntee that you're going to need to extend that time frame. The thing about the forums is that maybe 40% of attendees are on here, and everyone else just shows up at the con. D.Gray-Man is still hugely popular, and even if not a lot of people are replying here, there's going to be lots of cosplayers and fans at the con. Also, a lot of people upon getting to the con don't know about the gathering and hear about it through word of mouth, so that's even extra!
Even just making it an hour, I think that'd be reasonable : )