i'd imagine theres a number of days waiting period before they are deleted. Not everyone uses WiiConnect24 so there will be lots of consoles who arn't always online.
Thats even if there is an expiry. I doubt there is, altough if there is it will be quite lengthy i'd imagine.
All I know is having 30 miis turn up at once is so aggrivating having to close the window every time lol