The federal government asked the Supreme Court in February to clarify the government's powers to reform or abolish the Upper Chamber and the court will hear arguments this fall.
McLachlin says it can take anywhere from three months to a year for a judgement on difficult cases.
"We will do our best to get an answer for the Canadian people as quickly as we can, but obviously there's a lot of work to be done, a lot of thought to be put into the questions, which are not easy, and it's a huge responsibility," McLachlin said Saturday at the Canadian Bar Association conference.
"We have to make sure that we have digested all the submissions before us and that we've given each question our deep and most conscientious consideration, so that may take a little time."
She says the court will answer questions as to how change can be made and what is required in terms of consent by the provinces.
Ottawa has said it should be able to reform the Senate without reopening the Constitution.
There are questions about whether a majority of provinces have to be on side with reforms, or whether all of them must agree. But the Conservatives argue the high court can decide, in some circumstances, to allow the federal government to side-step the provinces in making changes to the Senate.
Calls to reform or abolish the upper chamber have grown because of a controversy over the expense claims of a handful of senators.
McLachlin says she doesn't know how tough the case will be.
"I really can't answer that until we get into it, then I'll know how difficult it is, but at this moment I just know that it's a case that's coming our way," she said.
"We have many difficult cases and we'll do our best with it."
McLachlin also used the conference to talk about access to justice, something she said is a growing problem for many Canadians. The chief justice says if people can't get access the system, they either don't resolve their problems or have to resolve them in other ways.
"The result can be increased violence, increased intimidation of individuals, it can be neglect, it can be having to live with your particular problem for the rest of your life instead of resolving it and turning the page," she said.
"People can be and are totally incapacitated by the fact that they can't resolve their legal grievance until it overcomes them and their lives are ruined. Their lives are ruined and their contribution economically and socially to society is diminished."
McLachlin says a lot of people don't get access to justice because they don't know where to go. She also would like to see action taken to ensure costs are affordable and that there are enough courts and staff to hear cases.
A full report on access to justice will be released this fall, but a summary report is to be released Sunday at the Canadian Bar Association conference.
McLachlin says it's important for the association and governments to act so the report doesn't gather dust on a shelf.
"Well I think the basic responsibility is to have enough courtrooms and judges and court staff to hear all the cases, to ensure that the costs of using the system are not so prohibitive that people don't want to or can not access it," she said.