I believe it should be none. If you sentence someone based on how they feel about their crime, you are punishing (or rewarding) people for their thoughts, rather than their actions. People are free to think as they please, and should not be punished for their opinions. We may not like them for what they are thinking, but fair and equal treatment under the law means the jerk gets the same sentence as the nice guy, when they commit the same act. I believe acting otherwise is a violation of the 8th Amendment, disallowing cruel and unusual punishment. I would argue that the 8th Amendment is specifically to prevent one man getting a different sentence than another, simply because of his status. I think the "and" is the important word: the punishment may be cruel (subjective), but it had better be meted out the same for everyone (objective).

Additionally, who is to say they're sincere? I would argue that those who believe they should get away with crimes because "golly, I'm really sorry!" are actually not taking full responsibility for their actions, and are probably not sorry. People "find Jesus" all the time and want to be released from prison. How is how ethical or fair? They did the crime, and they should do the time. If we're going to release people for finding religion, or sentence people for being "sorry," I can guarantee every criminal walking into court is going to be throwing himself at the jury's feet, crying and wailing about how extremely, sincerely sorry he is. Will we determine sentence length by some sort of sob-o-meter?