The length of trials varies depending on how complex the issues are and how long jurors spend in deliberations. Most trials are completed within a few days or a week. The judge knows about how long the trial will take, and he or she will tell you the time frame when your group is called for jury selection.
Judges know how difficult long trials can be. Let the judge know whether it is a serious hardship for you to sit on a long trial. Be patient during this process because a lot of people have similar time concerns.