So, why do I hear about Grand Juries that work on a case for substantially longer periods?

Grand juries serve different functions.

Grand juries are sometimes used to investigate crimes. The grand jury has the power to issue subpoenas, take witness testimony, consider additional evidence, and serve as an investigative arm to determine whether (and by whom) a crime has been committed. Some grand juries are empaneled to investigate either an entire criminal enterprise or even a particular category of crimes (say, investigating and organized crime family or the drug trade down by the docks), where they are expected to deal with many different interrelated cases that result in multiple criminal proceedings. Those processes can take some amount of time.

But grand juries are also just there to review and authorize indictments (including all felony indictments) that the prosecutor's office want to bring. There's always at least one grand jury empaneled that a prosecutor can run to when they need an indictment issued. That role can (and does) happen very quickly - it's just the prosecutor going into the jury with their case file and asking for an indictment.