General Inquiry: Skill Effect Duration Target

Trying to build around Shard scavenger a question popped up re: duration changes:

If a skill has 2 separate Durations, is there any way to know (without testing) which element of a skill a duration support gem or passive targets?

For example, Shard Scavenger has 2 durations. Duration of the buff during which you it auto-reloads (desirable) and duration between uses (undesirable). Both Prolonged and Compressed Duration can be slotted into that Skill gem. Obviously one would want the buff duration to be prolonged and the duration between uses to be shorter. There are also passive skills that prolong and compress duration. I will test the passive/gem's targets but wondering if anyone knows or it says it somewhere.

Thanks