Snow said that resolving where the remaining detainees would be moved to could also take some time.
"We have tried as best we can to move those who are in Guantanamo either to their home nations, or nations where they are wanted for other trial or justice dispensation," he said.
"But we also have laid down the benchmark that you also have to be able to assure that they're going to be treated humanely.
"Very few countries want these people back, and, therefore, what you have to do is to work through a procedure where you do, in fact, bring them to justice."
The United States has said it intends to try between 60 and 80 of the 385 foreign captives still being held at Guantanamo Bay, including 14 "high-value" prisoners sent there in September from secret CIA prisons.
Snow also acknowledged that Robert Gates, the defence secretary, had argued for shutting down the prison quickly shortly after he took office last year.
He expressed his opinion to Alberto Gonzales, the attorney general, who argued against closing the prison and moving the detainees to the United States.
"So whatever ideas the secretary of defence had coming in, when they had a discussion, he [Gates] deferred to the legal opinion of the attorney general," Snow said.
Condoleezza Rice, the US secretary of state, later told reporters that "everyone" wanted to close Guantanamo Bay but she did not know if this would happen by the end of Bush's presidency.
"The problem is that you have a number of dangerous people who, in some cases, cannot be held, cannot be tried in other places and they are too dangerous to release. So you need to be able to deal with it in some way," she said.
"The president has been very clear, he would like to close it, we all do ... We are all trying to work to make that a reality."