I can speak a bit from prior experience a couple of years back with jelqing. The thing about jelqing, and really any form of girth work, is that your goal should generally be to get as much expansion as you possibly can. That means the bigger and thicker your cock gets in any particular session, the more results you will eventually see.
Now the longer a session is (obviously within reason and more importantly with as much intensity as is SAFE) the more expansion you will see. Intensity of jelqing is key here (that doesn't mean squeeze so hard that you hurt yourself) combined with time.
Think of it like this, when you start a jelqing session, how much expansion do you get? Now compare that to after 10 minutes of a great session, and that to 20 minutes. Basically if you can keep your erection level consistent with the level that works best for you, you should be seeing your penis get slightly thicker through the session. So if you expand that session out a bit, it can benefit you.
WITH ALL THAT BEING SAID, THE DISCLAIMER SHOULD BE THAT THERE IS A LIMIT TO THE AMOUNT OF TIME YOU SHOULD SPenis EnlargementND DOING ANY KIND OF GIRTH WORK FOR SAFETY REASONS. For example, an intense technique like clamping generally should not go more than 20-25 minutes. So if you do increase the time, be reasonable.