I restarted slack about three hours ago, and as of right now it's using 552MB [0] across 5 processes. It goes back down if I restart it, and that was enough of a "wontfix" for the slack team when I reported it.
14 channel, I don't know how many there are on the server. Not sure on the number of messages / hour, but it is quite a lot. Leaving slack on overnight has my numbers up to about 800MB. Regardless of how many messages, 800MB for a chat client is nuts.
[0] http://imgur.com/S35NWij