As with other heirlooms dropped by Garrosh, this will only scale from 90 to 100. Once the 6.0 pre-patch for WoD goes live you will have a guaranteed heirloom drop for your spec on your first kill on each difficulty (Flex/Normal/Heroic - or Normal/Heroic/Mythic as they will be called then) if you have not received an heirloom for that spec prior to then.
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].
As with other heirlooms dropped by Garrosh, this will only scale from 90 to 100. Once the 6.0 pre-patch for WoD goes live you will have a guaranteed heirloom drop for your spec on your first kill on each difficulty (Flex/Normal/Heroic - or Normal/Heroic/Mythic as they will be called then) if you have not received an heirloom for that spec prior to then.
Also,
Source: Siege of Orgrimmar Changes in Patch 6.0 and Warlords of Draenor