NBA draft picks usually enter the league immediately. It is almost without question for players that leave college, including Kevin Knox, who the New York Knicks picked ninth overall in 2018. Others take years to arrive, however, and they are usually international players, including the three-year wait the San Antonio Spurs had for Manu Ginobili.
The Knicks have a short history of international prospects selected in the draft. One of them, Kristaps Porzingis, entered the NBA upon selection and played as a 20 year old in the 2015-16 season.
Some draft picks, whether from college or internationally, never play in the NBA, though, but their rights remain with the team that selected them.