Wendy Lyn, who runs food tours and the blog The Paris Kitchen, has this to say (and I agree with her; this is something I really had to learn when I first moved to Paris):
"> How far in advance should I make reservations?
"Each wine bar, bistro and restaurant have their own rules – some will take reservations months in advance, while others won’t. 2 – 4 weeks minimum are the norm and as soon as possible, especially of popular restaurants. Your French-speaking hotel or apartment concierge will be extremely helpful to help get you the reservations you need in time. A good rule of thumb is to make dinner reservations as soon as your plane ticket and hotel have been booked.
"Is it really necessary to make advance reservations?
"Everyone is vying for a table in Paris’ most popular restaurants – most rarely have more than 24 seats and receive several hundred phone calls a day. Reserve early to be on the safe side. Walking around looking for a decent place to eat when you are a foodie in Paris, is a gamble."
Check out the Restaurant FAQ on her blog, which is the source for the two Q/As I just posted.
Resto FAQ from The Paris Kitchen