The drop rate is about 2-3%.
Some people have gotten it on first try, others, in many days. I believe a THF with any TH will be helpful. Best farm off the lizards by Gemini Falls. There are a lot in one area, makes searching for them easier.
I did the quest for this scroll today. I had a lvl 70 THF with Thief's Knife with me. He also needed the marble. After about 30m or so we got mine. Five kills after that we got his. So we got two marbles in about 40-45m. If you can get a THF with TH3 or TH4 doing this quest shouldn't be too difficult.
Went down to Gemini Falls (you don't need to go through Ifrits Cauldron as I sadly found out later...) rounded about 10 lizards up and started hacking away at them. Had just TH1 and went 1/20~
On a side-note, this is also a good place to start leveling your Fellow.
Ok, so there is a lot of different views on the quest. As far as I'm concerned it pays off. The marble drop is rare... kind of. I spent 3 hours farming with a friend to get his, and it took exactly 1 hour, 3 min, and 27sec for me to get mine. (I used my 2 hr to make sure I didn't spend over 2hrs farming for it.) For the most part tho, on average when people farm they make about if lucky 40k-60k a hour anymore... this scroll is a nice relief from having to deal with the task of farming, regardless if you can only do it once.
