Hi,
Well no, it really isn't true that chefs sharpen their knives every time they use them, way too busy for that.
It depends on the type of knife and how often its uses. Different types of blades hold edges differently.
Serrated knives are rarely sharpened. Carbon steel knives may be sharpened daily and "steeled" before each use but it really depends.
There are so many different types of knives that there is no absolute answer to how often to sharpen a knife. Depends on the blade.
Here is a good discussion on knives in kitchens-
forums.egullet.org/index.php?/topic/2603...ance-and-sharpening/
Cheers,
G