It depends entirely on the court's schedule that day. I've been called in every two years like clockwork ever since I got a DC drivers license in 2000 or so. There have been times where I've sat in the main jury room until lunch, gone to lunch, come back and have them immediately say we can go home. There have been times where I've been called into multiple potential trials in one day but not selected. I think one time I sat in the jury room for an entire day and never saw the inside of a courtroom, leaving around 3 or 4. I also have been selected for one trial, which lasted two weeks.
If you get selected for a jury, expect days when you'll be there all day, and days where you go home unexpectedly early. There's really no way to tell ahead of time.