The higher a creature's level was.
Under normal circumstances, it would be less likely to be injured, or it would consume a large amount of its own energy. However, if it was injured or its energy was lost, it would usually take a longer time to recover.
Some gods were injured after fighting with creatures of the same level.
It might even take thousands of years for him to recover to his peak.
In the battle between garen and the Twin Flower oracles, he was not injured. Although the two demigods could coordinate and restrain him, they could not cause effective damage to his giant God transformed body.
Garen had to sleep for a long time to fully fill up the large amount of time power he had used to prevent the other party from escaping.
The speed at which he was absorbing the power of time was getting faster and faster, but the total amount of time power that his body could contain was also increasing.