It depends on what you are wanting to do there. If you want autographs/photos with the princesses, there will already be a long line. I would get there at least 30 min prior to opening if you want to be at the front of the meet/greet line.
like danger said if you want to do the picture thing get there early!
When we went in Nov. it opened at 9am. I got there in line at 8:20. we were the second family. But around 8:50 the line was pretty darn long. by the time we got out of there there was a line that was 1.25 hrs wait.
Is this actually a line to get into the Princess Fantasy Faire area or just to see the Princess's themselves. Can you just walk back into that area and look around, etc, without a wait.