I was wondering if there was a certain length for whipping, or if it's just decided by the sling maker, or any other system for deciding?

https://en.m.wikipedia.org/wiki/West_Country_whipping]

This^ I also use something similar to sail makers whipping frequently in sling making. Sewing an overhand knot or whipping to the main body of the cord is an iron clad way of making sure it will never come loose. I use it for more than just whipping. It's pretty handy. West Country Whipping is also useful at the end of a release cord for making it heavier and larger in diameter if that is something you prefer.

As for how much is too much, I personally don't use any whipping at the pouch end of the release cord. I prefer making a sling that doesn't have the sling stone hit that area enough to need it. Leather can be sewn on as is done with Balearic slings though if you're having issues. Ideally the stone should exit the pouch with as little contact as possible with the release cord, both for sling durability and accuracy.

Just my personal opinion , no. It says on the article it's prone to slipping. On a sling with all the movement only using that knot will mean it eventually comes loose. That's been my experience anyways.