It depends on the facts and circumstances of your case. Was the arm a firearm? Did great bodily injury or death result? In general the maximum punishment for robbery is 9 years and another 10 years if it is connected with a firearm. Sentencing rules may lower these outside totals somewhat.
As Mr. Kaman explained, without knowing the very specific charge and firearm enhancement alleged in the charging document, the range can be wide. California has several firearms enhancements that can add anywhere from one year (for a principal who is not the one that actually possesses the firearm) to 25-life for intentional discharge of a firearm during a robbery that causes great bodily injury.
To give you an example, without knowing the specifics of the case you're asking about: Suspect pulls a gun on the street, demands money from another person, takes their wallet and flees. Robbery is 2, 3 or 5 years in state prison plus 10 years for the gun. The 10 years cannot be stricken by the judge, so the minimum is 12 years state prison; maximum is 15. There are other gun enhancements that may be alleged or bargained for, greatly changing the sentence.
This is a question that requires more facts to answer, specifically what if any allegations are charged or if this simply a "straight" armed robbery. Since weapon enhancement creates substantial exposure under california state law, you simply can't answer this question without knowing more.