I assumed it was once per game.
Is the gain 2 Soldiers in any colony Building from the base game once per round? I thought this was once per game so I followed the same logic for Papal Edict.
The Native Allies building is once per game, immediately, and it says so on the tile. Papal Edict just says "gain 2 colonists..." leaving you to wonder if this is every round, in the Capital Building Benefits phase, or once immediately, when you buy the tile.
It is lack of consistency that makes the buildings confusing. For example, the British National Advantage is to "take $1 from each player." This is clearly once per round (it would be too weak otherwise) but that is not indicated anywhere in the rules. We are basically left to our own devices to decide what is reasonable. And that is really bad design.