For example, in New Zealand (where I live), prostitution is legal. It is legal to be a prostitute, to purchase the services of a prostitute, and to advertise your services as a prostitute (usually in the entertainment section, between the Garfield cartoon and the movie listings, which makes them hard to miss).
It's the same in Christian fiction. I am more forgiving of unchristian behaviour from a non-Christian character, particularly as the plot often focuses on that character's journey towards becoming a Christian. But I don't need the details. You can tell me that he swore without telling me each word. You can have a murder or a rape scene or a sex scene without the graphic details (I'm married. I know what goes where). However, if Christian characters are sinning, there should be some remorse, some acknowledgement that this is outside God's law.
So I don't think it's so much a question of 'how much' unchristian behaviour as how it is presented, and whether there is remorse, repentance and a move to a growing understanding of or relationship with God.
I would also add that some readers of Christian fiction don't want any of this 'unwholesome' or 'edgy' behaviour in their books, and that is their right. As paying consumers, we all have a right to choose to give our money to those authors we like best.