How often do harvesting spots recharge?

Scattered around the map, I've found a few harvesting spots where I can collect free resources. The tooltip says to "check back often" to see if harvesting is available.

I've found that they don't recharge daily or even at the same time. So how do they recharge? Is it a certain amount of time since they were collected from? Is it a certain number of battles fought? Is it random?

From my observations regeration rate is somewhat constant, but it's shared across all Harvestings Spots.

In the begining of the game, the two spots I had, were recharged almost every day, but now they recharge also almost daily, but it's often hard to find which one has recharged. I would also assume that recharging timer is 'ticking' faster then you play the game.

I also noticed that high-tier spots are recharging more often, then you are high level, so I would assume there are not 'purely' random.