Yssupgnitaeevol— I hang lower weights for longer periods, and there is a point in the process where, at certain weights, you might be able to prolong the time you can stay in the clamp, by not tightening the clamp so hard, but that ‘zone’ is a very narrow envelope, so the basic ‘rule of thumb’ (20 minutes per set) is a good one.
When you’ve been hanging consistently for a long time (maybe a year or so) and your penis is conditioned to hanging, it is possible (generally towards the beginning of your routine) to hang for longer than twenty minutes, but that comes with time and perseverance.
The most important thing is to keep checking your penis. For example, if you’re numb after just five minutes you should immediately discontinue the set (regardless of the general 20-minute guideline), however if you’re able to hang for half an hour at a time before you begin to start getting numb, then it’s probably okay to do so (the danger is basically in cutting off blood supply, which subsequently causes cell-death. While you’ve got plenty of feeling in the glans and it looks relatively healthy, generally speaking you should be okay to continue the set). Also— care should be taken to avoid damaging the dorsal nerves and the nerve bundle just before the glans (but that’s a different issue).
In addition, it sounds like what you’re talking about is using a hanger as an ADS. I imagine it may be perfectly possible to do so (providing you don’t tighten it enough to cut off the blood-supply), but best to measure (and keep a record of) your gains so that you can tell if the low weight is actually working.