I used to have a DD-WRT router with bandwidth monitoring but we're now using an Apple Airport Extreme, and I don't think it has such capability :/
If you have a Mac always running on the network, you can try freeware app http://www.thotzy.com/THOTZY/Thotz/Entries/2008/10/26_Traffi...
I used to have a DD-WRT router with bandwidth monitoring but we're now using an Apple Airport Extreme, and I don't think it has such capability :/