Am I understanding you correctly? You are required to serve 7x every 90 days?
No. It appears our county sends out jury duty summonses every 4 months. (I'll call that a "session" although I don't know what the real term is. Also, I re-looked at my summons and my first assigned day was Jan 9 and my last is April 27 so it's closer to 4 months than 3.)
When you are summoned, you are assigned seven non-consecutive days during that 4 month "session." If a case goes to court on one of your assigned days, you're part of the jury pool. If you are selected, you stay for the duration of the case. If not selected, you're excused till your next assigned day.
However, you are not called as a juror for every 'session.' I have no clue how they choose who gets summoned. We have lived here 20 years. I've been summoned four times, but my husband and now-adult kids have never been summoned. It hardly seems fair. (They are also registered voters.)
I don't think it's a particularly active court though. So far, 2 of my days have passed, and I haven't even had to go to the courthouse either time. We're supposed to call a phone number the night before we're scheduled. The first time, I called and was told that the scheduled case had been settled, so I was not needed. The second date, I got a letter a few days beforehand saying no case was scheduled, so I didn't need to appear till my next assigned date. Across all 4 times, I've only had to report to the courthouse a couple of times and I've never been picked for a jury. I'm hoping my luck holds! I do not think I'd enjoy being a juror!