Mueller is a special counsel appointed to continue a previously-extant FBI investigation into Trump's dealings with Russia. Unlike prior independent counsel/special counsel appointments, there's no limit to Mueller's scope. It's basically "look for wrongdoing." Ken Starr was appointed to investigate Whitewater, a minor land deal the Clintons had lost money on 20 years previously. People were already in jail. Nothing was active. And still, three years later we got taped depositions about whether or not Clinton had banged an intern. The investigation was still going three years after that. Watergate was a small, independent arm of Nixon's re-election campaign and whether the President was or was not aware of their illegal actions. The Washington Post had Howard Hunt's name in the Watergate burglars' address books three days after the break-in; it was six months before a special prosecutor was appointed, three more until hearings began, and another year before Nixon resigned. The Trump investigation, by contrast, appears to qualify as a "vast conspiracy" and we're barely six months in. The argument at hand is whether or not a corrupt real estate developer with ties to organized crime coordinated with an adversarial foreign power to disrupt the electoral process and install himself as a blackmailable puppet. From within that rather... open solution set the Mueller team has to come up with a definitive conclusion. There are no "mights" or "could haves". It's "there is evidence of" and "the record indicates that." It's gonna be a while.
Is the lengthiness of the process dependent on solidity of the evidence? you have to make sure you're getting all the right guys, no matter the hearsay?
I'm no expert, and this country hasn't done a whole lot of this. I think this is the fourth time since Watergate 40 years ago. I just know that the prevailing conventional wisdom is that even if Mueller weren't a methodical perfectionist with a great attention for detail we'd still be in for a wait.