In California, the legal landscape around marijuana has shifted dramatically since legalization, and that shift has filtered into how prison disciplinary systems treat it, though it is still contraband inside a correctional facility regardless of its legal status on the outside.
Additional prison time specifically for a marijuana possession infraction is unlikely in most California cases. The disciplinary response is typically handled internally rather than through new criminal charges, particularly for simple possession of a small amount. The penalty lands more heavily on the quality of his remaining time than on the length of it.
The immediate consequence is almost certainly the SHU. He is likely in segregation now and can expect to stay there for several weeks while the disciplinary process plays out. That means restricted movement, limited commissary, severely limited phone access, and the general misery of isolation. With eight months left on his sentence, spending a significant chunk of that in the SHU is the real punishment.
Beyond the SHU placement, he will likely lose good time credits as a result of the disciplinary finding. That is where the sentence length gets affected, not through new charges but through the reduction or elimination of credits he had already accumulated. How many credits he loses depends on how the infraction is classified and his prior disciplinary history.
Getting caught with contraband this close to the door is one of the more painful situations an inmate can put themselves in. Eight months is short time. The SHU makes it feel much longer, and the good time loss pushes the release date further out. Encourage him to keep his record clean from this point forward and serve out the balance without any further incidents.