When a person commits the crime of arson the minimum and lowest sentence for arson is 5 years in prison.
However most people who do commit arson get between 15 to 20 years in prison depending on if it's their first offense or second offense.
If it's the persons first offense committing the arson and they do no harm to anyone then they may get off with 5 or 10 years in prison.
But if it's the persons second offense or they intended to harm someone then they will usually get 15 or 20 years in prison..
If the person commits arson and then kills someone in the arson such as in setting a house on fire with the intent to kill someone then they can get 25 years to life in prison with or without the possibility of parole.
I've heard of a few people just getting probation when they've committed arson but usually that's for an unoccupied dwelling.
Usually though you will get at least some prison time for the arson.