You can use my Temporary Ability System which adds ability to unit for a certain amount of time and be removed automatically.
The idea is, to periodically pick unit group around the unit that has this ability, and add the ability (using my system) to the picked unit.
Of course to do this, you must add the learning hero/unit to a Unit Group for periodic enumeration over the time.
The duration of the added ability (my system) and the periodic timer determines the aura delay (getting/losing the aura).
I have created a test map, if you can take a look at it, you'll understand.