It can vary.
Slush recently made some changes to his mining pool that reduce the need for communication to much lower levels if you change your settings:
This feature is aimed to ASIC miners and big mining operations, because it drastically optimized communication overhead. You can have 1Thash/s miner connected to the pool and network communication will consume less than 10kB/minute...
Maybe other pools have this as well.
Thanks for this. Was interested why so many people are obsessed with setting up in server rooms if the bandwidth wasn't that high. All you need is cooling and power, which you can have in a shed :/
