For those of us with limited network connection Network Traffic Monitor (NTM) is a very helpful tool. It can monitor upload, download or total network traffic. It can also monitor time spent online and lot of other things. NTM is not available in Ubuntu repositories so you will have to download the .deb from here and install it. After installing start NTM from Applications->internet->NTM. Now right click on the NTM icon in the taskbar and select preferences from the menu that appears. This will open the NTM preferences window. In the General tab specify the interface that you use to connect to internet. In my case it is wlan0
Now select Traffic tab and enter the traffic limit of your internet plan. I have entered 2G i.e 2048MB as an example value.
You can select Auto Disconnect if you want the connection to disconnect as soon as you reach your limit. This can be very helpful if extra MBs are charged at a high rate. Click Apply and you are all done. NTM will monitor your internet use and you can left click on the NTM icon in Taskbar to see your current speed, bandwidth you have consumed and lot of other information.
If you have time limited connection and not bandwidth limited i.e you buy internet hours and not GBs then you can also use NTM to keep track of time you spend on internet. Open the time tab to configure time related options.