You can use System monitor to see the traffic used in a single session. Use
gnome-system-monitor to open it in a terminal.
You a totalistic view of usage, you can use
ipac-ng. Install it with the command
sudo apt-get install ipac-ng
Here is a good guide about usage of this software.
Alternatively, You can also use
vnstat. Install it wit the command:
sudo apt-get install vnstat
To start vnstat cron job, you would use this simple command:
sudo vnstat -u -i eth0
eth0 with the actual interface for you. It is
eth1 for me. This job will run every five minutes measure and update the usage data. At any given time you can use
vnstat command to see the usage.
A good guide about using vnstat is here. You can also check Ubuntu manual
page for vnstat