Last Fall, I did a 2-minute "routine" of a pullup every 15 seconds, so 8 pullups and a total of 2 minutes of being on the bar, using a normal diameter pullup bar that was pretty heavily knurled. I did this with no specific training at all, including not practicing pullups. I think it's actually more interesting than just hanging there, watching a clock.
Since then I do regular hangs for time, generally in the :45 to 1:15 range, and if I'm using a 12 kg on a belt, :30 to 1:00. I think I've done 1:15 or 1:30 with the extra weight. I train it in the form of passive hangs because those are good for my shoulders, and the extra weight helps stretch out my shoulders and also my spine.
A couple of points to take away from my experience, IMHO. First, if you're able to deadlift a reasonable multiple of your bodyweight, you'll likely have the grip strength to hang from a bar, anyway, and it may not be something you need to train. 2-1/2 times bodyweight is a good number to shoot for, and you may win a local powerlifting meet with that, too. That's 450 lbs. for a 180 lbs. man, 500 lbs. for a 200 lbs. man, or 375 lbs. if you weigh 150. 175 kg for a 70 kg man, 245 kg for 90 kg, etc. - you can do the math. For over 90 kg, perhaps a bit less than 2.5 x bodyweight. For women, call it double bodyweight. If you're older and you want to lower the weight standard, that's on you.
With that deadlift, you'll have good ab strength and a good grip.
And second, to broaden the discussion a bit, we're on to the "what the heck effect" - a few, well-chosen lifts in your training will prepare you to do a lot of other things with only a bit of training on those other things, and once in a while, no training on those other things.
As to what length of time to use in training bar hangs, always remember that volume is what you're after, and in this case, it would be total hanging time over the course of a period of weeks or months, so get off the bar when it feels like it's getting tough so that you'll be able to get back on the bar later in the day again, and again, and again. Push yourself a little once in a while, and test yourself every few weeks or so.
-S-