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

[KiloVictor]

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.
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This is the conventional wisdom (some say 3 seconds, some 3.5), but in fact there is a scalar factor which varies per spell. Also, all DoT/HoT spells receive full benefit of any +dmg/+healing items, but it is added incrementally to each tick of the spell. But to know the % of +dmg/healing that will be added to a given spell, you pretty much have to test it out and record the results.

I know on the official Warlock forums there is a post which has empirical values of the % scalar for each Warlock spell, and there are probably similar lists on the other fora.

~Kv