Tall cylinders are tough but good practice, and 15″ to 16″ out of one ball of clay is probably the physcial limit for most clays. It’s possible to throw taller than 15 inches from a single ball of clay, but you run into the law of diminishing returns.
It requires much less effort to throw large forms in sectionals or with coil throwing than to try and throw a 20″ or 30″ form from one ball of clay. The bottom simply needs to be too thick to support the upper walls at those sizes and you wind up with a ton of trimming needed and a much less delicate and refined form.
Several years back I was on a kick of throwing large forms in one go, and the best I was able to manage was 30 inches from a 43 lb ball of clay. It took forever, killed my hands, required a lot of trimming, and was absolutely hideous.
Throwing in sections, I was able to easily get a 4 ft tall pot, with no more effort than throwing 4 12 inch tall pieces. It’s just simply not worth it to throw much bigger.
I’m sure with everything perfectly prepared, right clay, right wedging, right water content in the clay, right hand positions, you might occasionally get past 16″ but you could probably take the same effort and put the work into stacking two cylinders to make something much taller, much quicker.
With that in mind, a few tips: Slow down your wheel on your later pulls. Dry your clay out as stiff as you can stand to throw. Wedge your clay completely, and then a bit more. Three or four pulls is usually the maximum, any more and you are just working water in to the clay and your cylinder will be shorter, not taller. Pay close attention to the second and third pull, and don’t hesitate to do a “half pull” that starts half way up the cylinder and moves up. Keep your fingers at 3:00 on the cylinder, if one slides away from the other you won’t compress and get a clean pull.
And of course everybody knows that a good India pale ale will help more than anything.