I did not see it, but I logged on after the snapshot had started. There is no reason it cannot be announced more than 2 times. I would think once at a half hour before the event starts, then at 15 minutes, and again at 5 minutes.
CM's are just trying to get a turn out to make the events fun, if it happened 8 or 10 times, then yea it could be annoying, but it is just text, I miss most of what comes up on text myself, but thats just me.