WoWDB

• 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.

Source: Siege of Orgrimmar Changes in Patch 6.0 and Warlords of Draenor