Basically, every 5 seconds:

  (6 * (75 + 125)) + (90 + 90) == 1380

There's 86,400 seconds/day and an average of 276 bytes/second (1380/5), so per day this is:

  276 * 86400 == 23,846,400  -or-  1380 * (86400/5) == 23,846,400

Monthly:

  23,846,400 * 30 == 715,392,000

Or, "a total of 715,4 MB per month", as the article states.