Speaking at his confirmation hearing before the Senate Committee on Armed Services, Miller warned against talking about "turning points" in the war.
"I can't guarantee you a timeline or an end date -- I know that going into this position -- or offer necessarily a turning point, unless there is one, unless there's something to report back and something has changed," he said.
Miller said Afghanistan is in crisis and challenges -- including Taliban territorial gains, Afghan force losses, food insecurity and political corruption -- need to be addressed.
"This is about protecting U.S. citizens, when you go right to the heart of the issue," Miller said.
Trump nominated Miller in the spring to take over for Army Gen. John Nicholson Jr.
Should he be confirmed, Miller would be the next commander of U.S. and NATO forces in Afghanistan. He is commander of the Joint Special Operations Command, in which he oversees Special Mission Units like the Navy's SEAL Team Six and the Army's Delta Force.
Several senators expressed support for Miller, whose confirmation is considered likely.
In August, Trump said although he thought the war in Afghanistan was too lengthy, "the consequences of a rapid exit are both predictable and unacceptable" and that a "hasty withdrawal would create a vacuum that terrorists, including [the Islamic State] and al-Qaida, would instantly fill, just as happened before September 11."