This is something that came up in one of my classes recently. Most sewers will purchase a separating zipper in the length recommended by the pattern. I always suggest buying a zipper several inches longer than what the pattern calls for. If you have to lengthen the pattern or make a full bust adjustment (which also adds length over the bust) you will need a longer zipper. It's so disappointing when you realize that your zipper is a couple of inches too short.
BTW, I have a love/hate relationship with separating zippers. It's often difficult to find the correct length and color so I started stashing (some would say hoarding) them years ago. I have hundreds! Friends call me and send swatches when they can't find what they need in stores. :-O Whenever I see zippers in colors that will be useful to me I buy them - in the longest length possible. That way I always have extra length to work with if I need it and don't have to be concerned about patterns calling for odd sizes.