I think this is what you are looking for
Q: How long do teams have to sign players once drafted?
A: The short version (the actual text of the details runs about 4 pages the way I have it formatted, and I\'m not posting all of that)
-- If the player is in the CHL (Canadian Major Juniors), the team generally has until the second June 1 following the draft when the player was selected unless the player leaves the CHL early; in that case, the team has until the fourth June 1 after the draft the player was selected. Details in Article 8.6(b)
-- If the player is in college, the team has until August 15 after the player\'s class graduates (example: a player enters college in Fall 2005, his class is considered to graduate in Spring 2009). Generally about four (4) years. Details in Article 8.6(c)
-- If the player is in Europe, generally two (2) years. Details in Article 8.6(a).
In any event, teams will generally lose all rights to an unsigned draft pick after the player reaches the age of 22.
The text comes from http://www.nhlscap.com/draft.htm