It is the quality of the exercise and the frequency that are paramount not so much the duration. Quality insures that the specific physiology is affected and frequency insures deformation or new size is being augmented. Duration (although, important) is probably the least critical of the elements to arrive at new size. Obviously, some duration is required to manifest gains but it does not mean marathon sessions are necessary.
So, how much is enough? If you are performing quality excercises than you can correlate PE gains to gains made in muscle development, in terms of duration. The good news is that the penis is so small (relatively, speaking), that the exercises needed to make it big are small in number (very small), this is a fact that gets lost very quickly in this sport. As a point of reference, your abdominal muscles might require 4 to 6 or more exercises to hit all the various layers and locations.
If your doing quality exercises you can do 3 sets of 12-15 repetitions (jelqs), this regiem could last anywhere from 12 to 18 minutes depending on your rest and time intervals. Your exercises will also depend on where you are on the PE calendar, you have “built-in” capacity, if you have reached capacity than different regiments need to be designed. So, much of the “time question” surrounds the quality of the exercises; playing your penis like a trombone is not a quality exercise (people will equate quality to the number of strokes like 300 strokes per session), this maybe good for your trombone it is not good for your penis.
You can get bigger and more toned biceps by doing the above cycles 4 or 5 times a week (using a dumbell), the consistancy or frequency becomes an important aspect of the muscle development. PE will work the same way, consistancy and quality are the cornerstones. You must understand your penis and the feelings of fatigue that signal a quality workout; know when to rest and when to “push it.” Time will always be a parameter of the PE activity but there are those that get a good workout during the time it takes to urinate and blow dry your hands. Get the QUALITY down and the gains will come. Hang long, hang tough. (this is a moniker, it means almost nothing)