Quote from radionoise;245156:
1~5%
When it comes to personalized enchants, either you get it on your first try or on the thirtieth.
You can also try advertising; someone might still have a leftover code from the Mystery Box event, though... the chances are pretty low.
It took me 5 tries to get mine. 3 were agonizing solo runs, the others were relaxing group runs.
Big rule of thumb - make sure you find people who are willing to give you their keys, so you can open all 3 chests. The enchant is personalized to whoever opens the end chest, so basically, open the chest, pick up only the enchant scroll, and then let everyone else get their gold.