There are many ways. But my favorite is to use a bookmark for the official usage site:
https://www.tatatele.in/prepaidPhoton/postUsage.do?mdn=<your photon phone number>&SOURCE=postpaidMCCI&d-4030679-p=1
This is for postpaid connections, for prepaid, I do not have any clue. If your accounts page shows the usage stats, you can open the frame in a new tab and get that link.
The only drawback is that its not real time. The delay is generally of 3hrs, but sometimes it may go upto 12 hrs, depending on the server updates. For real tme updates, I use:
Linux: vnstat with its php frontend. http://humdi.net/vnstat/ & http://www.sqweek.com/sqweek/index.php?p=1