PC 664 says attempted murder is punishable by life in prison w/o the possibility of parole. Given the other charges it sounds like you were charged in the alternative. If you were found guilty of assault with a firearm and criminal threats only the sentence would be considerably less. Obviously you need ab experienced lawyer to help you defeat and/or reduce these charges.
You have not provided enough information to give a definite answer. More is needed about the crime itself, and any prior convictions. At a minimum, however, a willful, deliberate and premeditated attempted murder carries 15 to life. That means that the person serves a minimum of 15 years before being eligible for parole, and could serve life. As a practical matter, it has been rare to receive parole on a life sentence in California for many years, although it has been occurring more in the last few years. Assault with a firearm is 10-20-life, depending on whether the firearm was used, discharged, or inflicted great bodily injury. Special rules come into place when there are multiple charges. The bottom line is that it is not pretty.
Mr. Brewer is correct it is not pretty. You asked for the max so here is a view of what you may be looking at. #1) Mr. Brewer is correct the base term is 15 - life for the attempt murder. Then on top of that even if the assault with a firearm arises out of the same facts (Penal code 654 prohibits multiple punishment for the same facts in some cases, you will receive a separate sentence for using a firearm on this count. The amount of time depends on whether you displayed it, fired it, or actually shot someone. If you hit them with the bullet they could take on another 25 to life. The felony criminal threats could also be a separate sentence that is served either consecutively or concurrently depending on the Judges view of the case. You could be looking at a lot of time if convicted of the charged offenses. Possibly in excess of 40 years depending on who the Judge is that is sentencing you and dependent on how the firearm was used.
Life with the possibility of parole. The minimum eligible parole date depends on the other factors in the case. Personal use of a firearm adds at least 10 years, personal and intentional discharge adds 20 years, and personal and intentional discharge resulting in death or great bodily injury adds 25 to life, all consecutive to the underlying Attempted Murder. The max is unclear, but it is not pretty.