To maximize your plunder in ebs as a hitter, skimming your troops is the optimum. Best payout is reached by doing 2 att / 3 assa per 5 minutes. Spy skimming can be neglected for hitters as those won't earn you decent plunder anyway.
This can however only be done if you are permanently online. So you will have to adjust your skimming scheme to your online activity. 4 att / 6 assa per 10 minutes or 6 att / 9 assa per 15 minutes are quite popular.
To your question specifically, a 30 minutes skimming will bring you more gold than full unloads, if you can connect every half hour of course.
PS: for spy builds, the skimming is valid for your spies (3 / 6 / 9 assa per 5 / 10 / 15 minutes) to maximize earnings. No need to skim att troops for spy builds.