It would really depend on your intentions. If you just want to get in, then you may not even have to get there until opening, or perhaps a little after (but not too long).
If you want to be among the first in, you'd want to line up early. As a comparison, for super soap weekend at the Disney/MGM Studios, people (not me) line up at 5 and 6 am outside the gates. While the park opens at 9:00, they let people in when the crowd numbers become unsafe, which can be as early as 7:00-8:00'ish at the Studios. That's just an example. I'm not sure about DLR's policies on that, but if they're similar to the Studios, then you may want to arrive that early. Or possibly even earlier as I'm sure that day will be very busy. As I said, it really just depends on your intentions.