Got my first and only Gacha 6 star at about level 80 around 40 days into the game. Viola on a 10+1 when she was featured.

The chance of getting at least 1 6 star on a 10+1 draw is just under 10% (I get 9.99913% from .995^11) so if you have one after 10 10+1 draws, you are on par.