Something around 6'-8'. Hook the leash to the pups collar as normal, run the leash along their back and then loop it around their belly right in front of their hind legs and create a choker loop. As they walk, if they tug and you simply maintain a grip on the leash, it will cinch the loop around their midsection, which will be uncomfortable but not painful to them. Typically they will stop to check and see what the heck got them, then carry on. DO NOT JERK ON THE LEASH! You should not use the cinch to correct behavior. Simply tugging on the gently is enough to get their attention. Idea is to set up a negative feedback to them, similar to a choker, that coincides with their undesirable pulling.

Over some time, probably a week or two, they will learn the appropriate pace to prevent the cinch from tightening and to stop once it starts to close. You can reinforce your STOP command by stopping, allowing them to continue and say STOP and gently tug the leash to provide feedback.

I was very successful with our tugger using this method. I learned this method from a professional dog trainer.

Yes, I know I'm taking a risk with this group using words such as "tugger" and "pulling", but it's worth the risk.