The sweet spot seems to be around 50-60ish skill. Depending what items you plan to be using when you're finished, you'll have to adjust it. (And if you want to go for the full 120, you'll just have to do it the hard way as your real inceases past 60.

Below 50, you want to use a combination of jewelry / bandana / shadow dancers / and rabbit form. At very low skill, you just fail too often and gains are very slow.

As you approach 60, you want to start removing armor pieces and jewelry. (You'll want to stop using rabbit form first. Rabbits walk slower than humans.)