How often can the templar heal?

Running around as a demon hunter, my templar is the perfect complement. He's definitely saved my life more than once with his heal ability. However, there have been times where he hasn't done it as I run around waiting for it to happen. Normally, this happens if he recently healed me (so I assume there's a cooldown on the heal). But I've also noticed that sometimes he heals me twice in quick succession, which makes me doubt that. Does anyone know what the limiting factors are on when and how often your templar will heal you?

The cooldown on the Templar's heal ability is 30 seconds (Source).

He also has an ability called Guardian which heals you as well and is also on a 30 second cooldown. If you have chosen both of these skills for the Templar, you should be healed up to twice every 30 seconds.