How much time can a 20 year old man go to jail for having sex with a 15 year old girl?

Lawyers are not here to answer hypothetical questions.

That depends on some factors. Obviously that is a criminal sexual conduct charge which carries some significant prison time plus registering as a sex offender. His prior criminal history, if any, will play a part as a lot will depend on what his sentencing guidelines are and what charge he eventually ends up getting convicted of or pleading to.

He could be charged with a violation of California Penal Code Section 261.5 statutory rape. 261.5. (a) unlawful sexual intercourse is an act of sexual intercourse accomplished with a person who is not the spouse of the perpetrator, if the person is a minor. For the purposes of this section, a "minor" is a person under the age of 18 years and an "adult" is a person who is at least 18 years of age. (c) any person who engages in an act of unlawful sexual intercourse with a minor who is more than three years younger than the perpetrator is guilty of either a misdemeanor or a felony, and shall be punished by imprisonment in a county jail not exceeding one year, or by imprisonment pursuant to subdivision (h) of section 1170.

That depends on a lot of things. I'd need to know the specific charges to give you an accurate answer. Even then, it's impossible to guess how much time someone ACTUALLY WOULD get if convicted. Obviously, I have no idea whether or not this person has an extensive criminal record or is on probation for any other cases.

In Michigan the person can get up to 15 years in prison and MUST go to prison. There is no probation for this.

Longer than he wants, I assure you. He needs to contact a lawyer.