I've only been involved in one grand jury case (a federal tax matter in New Jersey), but wondered how the hell they find people who can serve on these. It seems insane, but I don't think it is every day. I think the jury is impaneled for that length of time, and you can be called in any day, but they don't actually present evidence every day. In my case, it was only 2 days, separated by a couple months (but I'm sure they had other cases going on.)
Remember, the purpose of a grand jury is not to decide the outcome of a civil or criminal case (liability, guilt or innocence, etc.) This isn't a trial in court, as with regular jury duty. Rather, you are presented with evidence in a very one-sided proceeding, and are asked to decide whether the prosecutor has sufficient evidence to bring charges. Its a closed-door proceeding, no lawyers (other than the prosecutors), no judge, no cross-examination. There's a famous quote (can't recall the origin) that a prosecutor could get a grand jury to indict a ham sandwich.