A year into Mueller's investigation is there any end in sight?

As the special counsel investigation into Russian interference reaches the one year mark, the president’s legal team is publicly pressing for Robert Mueller to wrap up his probe.

Even Attorney General Jeff Sessions, who is recused from the Mueller investigation, recently opined that “this thing needs to conclude.”

But how much longer is the investigation expected to last?

When looking at the timeline of the investigation, legal observers say, it is important to consider that it predates Mueller’s appointment. By the time Mueller was appointed in May of 2017, the FBI had already been running a counter-intelligence probe into Russian interference in the 2016 election, so a significant amount of investigative legwork had already been completed.

The criminal complaint in the George Papadopoulos case says the FBI’s investigation “was opened and coordinated in Washington, D.C. and had commenced in 2016.”

By January 24, 2017, just days after President Trump took office, National Security Adviser Mike Flynn was interviewed by FBI agents. Three days later, FBI agents questioned George Papadopoulos. The false statements both men made in those interviews sealed their fate to become the first defendants convicted in the Mueller probe that would be launched nearly four months later.

Only ten days after Mueller was appointed, prosecutors presented a 22-page affidavit for a search warrant in federal court in Virginia to get evidence from Paul Manafort’s storage unit. This “suggests that Manafort already under investigation when the Special Counsel took over the reins,” as Judge Amy Berman Jackson, who is presiding over the Paul Manafort case in the D.C. federal court noted in a recent order in the matter.

“In the typical life of federal criminal investigations, it seems that this undertaking has already moved at warp speed,” according to former U.S. Attorney spokesman Randall Samborn.

Samborn is a veteran of several lengthy public corruption cases who served as a spokesman for Special Counsel Patrick Fitzgerald during the investigation into the leak of the identity of covert CIA officer Valerie Plame. He says even with the rapid pace, there is likely much more to be done. He noted the stack of criminal charges, guilty pleas, cooperation, and two trials that are scheduled in the coming months.

While the president’s lawyers urge for a speedy conclusion, any delay in wrapping up the probe is not totally attributable to prosecutors, former federal prosecutors say. Defense counsel can play a significant role in slowing down investigations. Good defense lawyers will often want to negotiate terms with prosecutors and those negotiations take time.

Criminal defense attorney Shanlon Wu, a former member of the legal team for Rick Gates who pleaded guilty in February to charges in the Mueller investigation, says delays can be helpful to the defense. “Speaking generally and not about any particular case, delay generally favors the defense as the passage of time degrades witness memories and the availability of evidence,” Wu told ABC News.

“If Trump and his legal team want the investigation to end, they could help advance that cause by answering whatever the special counsel and his team have in whatever setting is agreeable to both sides and doing it expeditiously,” Samborn added.

The issue of whether Trump will submit to an interview has been the subject of on again, off again discussions between the Special Counsel and the president’s lawyers for months, with recent suggestions that the president could resist subpoenas, possibly assert his Fifth Amendment right against self-incrimination and is beyond the reach of an indictment as a matter of existing Justice Department policy.

President Trump has also had several changes in his legal team, including the departure of his attorney John Dowd and the imminent departure of Ty Cobb. New additions to the team include attorneys Emmett Flood and Rudy Giuliani. “Every time you change lawyer, you start the clock. That part of the delay is absolutely attributable to the president. I’m not saying it’s improper, but the notion that the government entirely controls the timing is untrue,” one former federal prosecutor told ABC News.

Past investigations suggest an imminent end is unlikely.

The Plame investigation took three and a half years to complete. The sprawling Whitewater investigation that began as a look into President Clinton’s involvement in an Arkansas land deal and led to Monica Lewinsky and an impeachment trial ran more than six years. The Iran-Contra investigation lasted eight years.

Some legal observers point to benchmarks that could indicate the investigation could be nearing its halfway point. A special grand jury, like the one in Washington, D.C. that has been hearing evidence in the Mueller investigation, is typically empaneled for 18 months with a possibility of a six-month extension.

However, a new grand jury could be called, further extending the timeline. Former federal prosecutors say it is not unusual for complex and lengthy investigations to span multiple grand juries.

The special counsel has not indicated whether his team is nearing the middle or the end of the investigative process.

According to a Justice Department official familiar with the matter, people recruited to work on the Mueller team were asked to make at least a two-year commitment to the assignment.

Former FBI Special Agent in Charge and ABC News consultant Steve Gomez says such a time commitment would not be unusual for an investigation of this nature.

“When putting together a team for a case of this nature, prosecutors would want to have some continuity and would ask for people to prepare for a two-year commitment because investigations can take many months and commonly last two years or even longer,” Gomez said.

“It’s breathtaking for an investigation of this magnitude to be moving this quickly and I suspect there’s still a long way to go,” Samborn said.

ABC News' Trish Turner and James Meek contributed to this story.