A. A citizen selected for petit jury duty serves for the length of one trial however long that trial may run. The average length of trial is two to three days, although in some cases it may run longer. If you have not been selected for a trial, your service is completed at the end of one day.An average day on jury duty lasts from 8:30 a.m. to 4:30 p.m. Back to top.he average length of trial is two to three days, although in some cases it may run longer. If you have not been selected for a trial, your service is completed at the end of one day. An average day on jury duty lasts from 8:30 a.m. to 4:30 p.m. Back to top.