depends; is it a pre-paid event like Cinderella's Royal Table or something similar?
Or is it one you get dinged with a $10/per person charge if you don't show up?
If the former, I seem to recall 48 hours in advance.
If the latter, one day prior. But you can always modify the date/time to a date in the future and then cancel it if you don't end up with enough time.