It would be extended if there was one day's worth of material left to look at, or if the prosecutor was done but there was a small chance that something else could come up. In other words, inferring much from this is pretty hard.
FIL was on a grand jury years ago, for years. They finally handed down a true bill against a prominent politician, but they'd not meet for months at a time, then meet often for weeks. Depended on the flow of the investigation, how much time the prosecutor had for that case, what new leads came along, how much information had piled up, whether the prosecutor needed the jury for something. The old FIL was happy to vote to indict, but also knew he only saw the accusatory information. Grand juries do not have to see exculpatory information, and there's no rebuttal or defense. That's why they just limit themselves to handing down a true bill, saying that they think they've heard enough evidence against the defendant that he probably committed a crime and an indictment should be brought. To the extent they're a jury presiding over a trial, the only thing on trial is the adequacy of the prosecutor's evidence. A true bill does not need to result in an indictment--somebody else can come along with evidence that knocks that true bill off its pedestal. Mostly the grand jury was there because it was easier in some cases to get them to issue the subpoena.
You want the same jury from start to finish. Empaneling a new one would be a distraction and slow things down. Unless the prosecutor thought it wasn't going his way, in which case the distraction is necessary to remove an impediment.
If there's nothing for them to do, they simply don't meet. If Mueller decides next Friday he's really done, it can be discharged. On the other hand, since keeping it juridically alive is cost-free, there's no reason to dispose of it unless he wanted to announce that he was done and there was no chance of pulling in anything else.
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