"Increases healing done by spells by up to XX."

[hiryuu1115]

I believe the current understanding is that the benefit is fully applied to spells with a base casting time of 3 seconds. As casting time decreases, the benefit decreases correspondingly down to half the benefit for a 1.5 second casting spell. Channelled spells and Instant cast spells are 1.5 second casting spells for all intents and purposes.