The General rule of thumb is:
90 Days in advance if you want to go to a character meal or a popular restaurant such as Cindy's, Cal Grill, AP, Le Cellier, Sci Fi, 50s PT.
You dont have to make the PS for the other locations that far out but have a few made before you arrive. You can always change them later.
The exception to this rule is holidays. Then 90 days in advance for pretty much everything.