I have heard of people lining up about an hour ahead. I think that it all depends on the night and time of performance you are thinking about. The closer you get to Christmas, the harder it's going to be to get in!! I went to a mid-Dec 8:15 show and the line with the package was starting 40 minutes before the show!!!
If it's early in the month, 30-40 min is plenty of time if you want a "prime" seat. If you don't care where you sit, you can show up 15 mins out, particularly on weekdays.
The closer you get to Christmas, the more time you'll be standing in line. And if you're trying to see the last show on Christmas Eve, forget it! You'll be in line forever, with no guarantee!
Friday and Saturday nights are pretty crowded. We tried a Saturday night one time the first weekend in Dec and the standby line was a mile long for the 8:15 show when the 6:45 show started.