According to (and I wish I was making this site up) shiftschedules.com:
"Since laws need to be enforced 24/7/365, most law enforcement agencies and correctional facilities use rotating shifts where officers work in regular patterns of days on and days off. Sometimes the shifts rotate and other times they do not. Usually police schedules have rotating days off, but not always. Both 8 hour shifts and 12 hour shifts can be easily scheduled to rotate days off and/or to rotate shifts. The use of 10 hour shifts is appealing because it can provide 3 days off each week, but since 24 can’t be divided evenly by 10, excessive shift overlap is required to fit three 10 hour shifts into a 24 hour day. If your officer coverage needs are less for a few hours a day, a combination of 10 hour shifts and 8 or 12 hour shifts can be used to adjust staffing levels to fit your needs. Another approach to a 4 day, 40 hour week uses 2-12 hour and 2-8 hour shifts a week."
So...8-12 hours, depending on station, it looks like. Can't do much more without having, y'know, specifics, and something tells me this story would best be saved for an "off the record" telling.
-A