Legal Question in Civil Litigation in California

That is up to the judge. Once you are subpoenaed you are under the court's jurisdiction and control. The usual practice, however, is not to require non-party witnesses to show up until as close as possible to when they are needed. Prior to the start of trial the judge usually asks the lawyers to come up with an estimated schedule of witnesses and arrange testimony to fit with the schedules of non-party witnesses as much as possible. You should discuss this with the attorney who subpoenaed you. There is also nothing prohibiting you from talking to the other attorney as well to get a sense of how long their questioning of you might go on.