Well, if one is triggered by breaking an orb, it'll fall within a day, at most, and you've only got a 50% chance of it happening. And it should fall somewhere around midnight, I think:
At night, if the time is > 16200.0 and WorldGen.spawnMeteor is true, it'll set it to false and call WorldGen.dropMeteor().
So if a meteor doesn't fall after one night has passed after you broke an orb, then you probably did not trigger one and it should be safe to move on to another orb. Note: meteors that path into near players, NPCs, or chests, will go "Oh, nevermind" and try to find a better location. If they fail 100 times, the meteor will be cancelled. Permanently. It will not try to land again the next day, the meteor will just not ever land. I have to wonder how much trouble it might be to place chests along the surface such that anywhere a meteor would land would be too close...
That said, you can't ever run out of meteors after you've broken a shadow orb. If WorldGen.shadowOrbSmashed is true (and netMode != 1 (1 is used for the client in multiplayer, at least - I don't know about singleplayer), there is a 1 in 50 chance every day at sunset of Worldgen.spawnMeteor being set to true, which if I got the time correct in the previous thing should then cause a meteor to fall at midnight.