That would really depend on which IM networks you're using. FWIW, although it's hard to compare, my friend worked out Skype's usage for his desktop, and figured that it would use >2GB just sitting idle, in a month. While this was on 15Mbps connection (for which I believe Skype increases its BW usage), Skype is still going to be something you won't want to leave running all of the time, most likely.