The Vegas Golden Knights will have to draft and make some big trades in order to make the playoffs. Looking back at old expansion teams and how long it took them to make the playoffs, five years is not totally unreasonable.
1993: Anaheim Ducks (4 Years)
The Ducks joined the NHL during the 1993-94 season. In their first season in the league, they finished in 21st (out of 26 teams). While that is not amazing considering their best player was Terry Yake, it is not a shock.
The next season they dropped down to 24th in the league.