"138KB * 24 * 3600 / 5" should be 2.3287GB per day. And it's 2.3287GB * 30 per month.
Update 2: "For comparison, a 5-minute check would be considered a pretty aggressive checking interval, and would only consume 1,37 MB per month. Instead, TP-Link goes through the same amount of data in just 82 minutes."
This assertion from the article has multiple errors too.
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The whole argument of the author is built on a flawed calculation by the author and the author exaggerated the number by a factor of 10.
715MB/month in the title and the article should be 71.5MB/month according to other information provided by the author.
According to the author, "TP-Link product is using about 138 KB every 5 seconds — or 23,85 MBs per day — on timekeeping."
23,85 MBs per day is not right, because 138KB * 24*3600/5 is about 2.328 MBs not 23,85 MBs.