In the Tyranids expansion there is a Chaos card called Roghrax Bloodhand.

It has an ability that reads: "Bloodthirst- During a combat round in which 1 or more units have been destroyed at this planet, double this unit's ATK."

So how long does the effect last? To me it sounds like that the ability triggers during a combat round. Does it finish after just 1 round? Or does it activate until the end of the battle at that planet? Or end of the combat phase? Or even better- once it's triggered, does it remain active till the rest of the game?

I would think just for 1 combat round, since it says during a combat round. So I figure after the round it's done, then the ability goes away until triggered again.

I would think just for 1 combat round, since it says during a combat round. So I figure after the round it's done, then the ability goes away until triggered again.

This is correct. Since the ability (which is a conditional continuing effect, not a triggered effect) is only active during combat rounds in which a unit has been destroyed, it will stop being active each time the current combat round ends -- and will not become active again until a new unit has been destroyed in the current combat round.

Said another way, since the ability is only active "during a combat round," it cannot last any longer than the duration of a single combat round. Whenever a combat round ends, so does the application of the effect.