Once a player is drafted by an NHL team, he must sign a contract with that organization within two years. The exception is when a college player is drafted. Unlike the NBA, the NHL allows a player to finish his college career. The NHL team holds the draft rights to a college player until 30 days after he has left school.
North American players becoming 18 by Sept. 15 and not older than 20 by Dec. 31 are eligible for the NHL draft. The 20-year-old age limit does not apply to European-born players; they must participate in the draft if they wish to play in the NHL. The same two-year signing period applies to Europeans.
A player who chooses not to sign, or is not signed by the team that drafted him within the time limit, is eligible to go through the draft process again. The unsigned player must be 20 or younger when the draft takes place to participate. Players older than 20 can become unrestricted restricted free agents.
A player cannot be drafted more than two times. A team that is unable to sign a first-round selection within two years is eligible to receive another draft pick as compensation.
via Al Cimaglia Blog – Blackhawks opt out of three draft picks – ESPN Chicago.