It's a great thing, I should do something like that too!^^
I think every once or two weeks really depends of your schedule, maybe a mix of these two?
Regarding what to draw, if you draw Bienie everytime, you sure will get used to draw her even more, but at the same time you'll soon get bored trying to find idea to match. In the other hand, if you pick totally random characters, you'll have a lot of diversity, and sometimes some challenges!
Sooooo.... if you don't know what solution to pick, do a big mix!