More than you would guess! I mainly use WiFi (need a strong signal I'm finding) but have used mobile data some. When you do try it, Data usage in system settings gives you a graphical display of cumulative use over a time period which you can select/adjust. It also shows below this bar charts of data use by app. For me, mail use- even with mail app set to d/l only headers and no pixs- is 40-50% of Total. Example: from 1/18/14 to 2/6/14, I used about 36.3 MB in about a total of 4 short sessions. 17.8 MB was for e-mail, mainly headers (seems high, only d/ld the full message for a few) and 6.5 MB for the stock browser. I just found out if you tap the bar for each app shown, you can view the app settings and also restrict that app in getting 'background' data and to only use Non-mobile networks if available. Need to try that next time I do mobile data use.
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