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I'm not on an iPhone (or even on a smartphone - so they won't threaten to throttle me), but I'm currently at 13.9GB with 10 days still remaining. Last month I used a tad over 50.9GB and 10.4GB the month before that.
 
Roughly around 2.5 to 5 GB. It's impossible for me to stay below 2GB as Wifi can be hard to get a hold of.
 
I realized that I tend not to go over the 2GB threshold. The most I've been to, I believe, is just under 3GB and I seem to average around 1-1.5GB. I should be ashamed for not utilizing the full potential of my unlimited plan...
 
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