U.S. President Donald Trump says many things that aren't true, but his repeated assertion during a press conference that "it does take a long time" to "pull off complete denuclearization" stacks up pretty well. It will probably take a decade at the very least.
It's not hard for a nation to dismantle or destroy its nuclear weapons and related technological infrastructure. North Korea may have as many as 50 weapons, and could probably dismantle them in a few weeks. Destroying test facilities, including underground test sites, as well as centrifuges and other technology, would take a little longer. But the really hard part of "de-nuking" is managing to do it in such a way that other parties can verify your actions are legitimate.
And that presents a host of difficult challenges. To begin with, no one really knows how many nuclear weapons North Korea currently has. If it dismantled 25 or 30 in front of international observers, no one would know how many it might have stashed away in its estimated 10,000 underground shelters. Nuclear experts from the International Atomic Energy Agency could inspect all known nuclear facilities, examine their records, and test these for consistency against the known history and technical capabilities of the facilities. But records can be altered, or faked.