Playing on a gigantic map with lots of planets last night, I got the (mega?) event that places spies everywhere. I ended up with 26 spies spread around. One spy on almost every planet. Right now, I'm in the middle of one of those economic booms, but even with almost 1,000 BC going to espionage every turn I'm already hitting the 4-5 turn mark. And there are still 15 spies left! At this rate, I'll be killing them off until the end of time, and I'll never be able to use espionage myself.
I realize that the event has to be rather large to have any effect, but shouldn't there be some sort of practical limit?