How much time do you serve on a 20 year sentence in state prison?

Twenty years.

A prison sentence is typically issued in two parts, the minimum (the time you must serve before being eligible for parole) and the maximum (the time you must serve--statutory time--before you would be discharged).

If the minimum is twenty years, and there is no "Good Time" law in your state, then you will serve twenty years before being considered for parole. With such a long minimum, it is highly unlikely you will receive parole your first time up. It is most likely that you will serve until the halfway mark between your minimum and maximum.

If the maximum (statutory time) is twenty years, refer to your minimum and check your expected release date.

If you were given a flat twenty year sentence, you will serve twenty years, provided there is no Disciplinary Credit awarded in your state.