Commander-type Heroes can gain the abilities of Inside Informant and Insight, which give some amount of "Leech" to Dust and Research, respectively. I assume this means they can take a little Dust or Research from other civilizations, but how is the amount calculated? Does it change if the Hero is a fleet commander or system administrator? Does the Hero need to be near the borders of another civilization, or orbiting around one of their worlds, or what?
The description in-game says that you gain +0.25% * HeroLevel or +0.5% * HeroLevel of science (or Dust) per turn from enemy and ally systems in which they are located. (I think they stack, since each ability is described as + 0.25% or + 0.5%, so if you have both, then it's 0.75% * HeroLevel.)
So: the Hero must be a fleet commander in somebody else's system, and they get a pretty tiny fraction of the dust or research the system produces (Say, 15% for dust or research if you're level 20 and have both abilities). If it's an enemy system, then you're already invading it (or about to be killed by a counterattack), so it won't apply for very long. If it's an ally system, then you're probably better off using that hero elsewhere. It can be useful if you're not at war, and just killing time with the Commander-in-Chief ability to train your fleet while waiting around in an allied system to steal some dust or research, but that's specific enough to not come up very often - even then, that hero is probably going to be better for your empire as a system administrator than as a fleet commander. A mere 5 Wit lets your hero grant a +10% bonus to research and dust to one of your systems, which is more than the Leech is likely to do, and generates other bonuses to industry, food, and possibly other things depending on your hero's abilities. If you care about generating Dust and Research, you'll generate more of it with the various Wit-boosting abilities than with Leech.
Unfortunately, the effects of the leech are not listed in the summary-of-all-hero-effects in the upper-right of the hero inspection screen, even when that hero is a fleet commander orbiting an enemy/allied system.
In general, there is a small note at the bottom of the ability description that quantifies it. You can check abilities and effects in the top-right pane in the Hero Inspection panel.
IIRC, Leech is based on a small constant multiplied by the Hero's level.