How long should a pant fly and zipper be?
Like pockets, pant flys are both functional and visual elements. The main point and functional purpose is to be able to get your pants on over your hips, but then be able to cinch them in to fit your waist. Do not compromise on this functionality.
There is a visual component to flys too, though. The topstitching is seen on the outside which means it becomes part of the creative design as well. On denim styles, the topstitching is often in a contrasting color that makes the fly more noticeable.
Pant zippers need to be long enough to open the waist enough to fit over the hips, but not so long that it adds bulk to the crotch area.
The length of pant zippers is one of those details that when done right, isn’t really noticed, but when done wrong, can be a big inconvenience to the wearer.