How often is Network Information In Activity Monitor Purged?

I'm assuming it is on reboot, but wanted to know if I could change this.

For instance, can I keep track of a months worth of I/O on the network? Or does anyone know a way to write a script to do this at terminal level, like crate my own log?

I'm sure there is software that can be installed, but I'm a minimalist and like to keep things lean ... ;-)