I've always interpreted that to mean the entire reporting period. I tend to look at these measures real conservatively.
You do not need to have it active for the entire reporting period, you just need to meet the measure. If you turn it on two days before the end of the reporting period but can meet the threshold it does not matter.
Turn it on and let the good times roll!
While it may not matter in practicality, look at Page 239: "Response: Fully enabled means the function is fully installed, any security measures are fully enabled, and the function is readily available for patient use...For an EHR reporting period in 2015, the capability for patients to send and receive a secure electronic message with the EP was fully enabled during the EHR reporting"
"...fully enabled during the EHR reporting" Not to be mistaken with "...fully enabled during the ENTIRE EHR reporting."