Network Monitoring Tools

Hello all, and thanks for your help in advance. I am a college student who is part of a team that is designing a network for a church in our area. Tonight we are going out for a site visit to install some network monitoring utilites to get a baseline of their network usage over a two week period. We want the visit to take no more than half an hour.
What I am after is a simple to use and install network monitoring program that can be used to log activity over the two week period on Linux (version unknown). What can we use that is simple to setup and install? Thanks