With the release of Patch 6.0, the drop rate of the Heirloom weapons from Garrosh Hellscream on Normal, Heroic, and Mythic difficulties will be significantly increased.
Players who have yet to get any Heirlooms will have a 100% (guaranteed) chance of getting a spec-appropriate Heirloom when defeating Garrosh on Normal difficulty or higher.
The chance of receiving additional Heirlooms beyond the first will also be increased.
When Warlords of Draenor is released [on November 13, 2014] and players are able to head to Draenor and level past 90: Garrosh Hellscream will no longer drop Heirloom weapons [and armor].
Source: Siege of Orgrimmar Changes in Patch 6.0 and Warlords of Draenor