My friend with an iPhone 4S found he had reached his data limit (512mb/month) in just one week when he picked up the new 4S. Mind you he was probably using it more to play around with at first, but he does use it for email and texting a lot. He upgraded to a 6 gig plan.
I was surprised how much data it uses. I guess its using data for any question you have, even if its about content on your iPhone. Anyone else notice this? My use if Siri has all been on wifi.
“If you are neutral in situations of injustice, you have chosen the side of the oppressor. If an elephant has its foot on the tail of a mouse and you say that you are neutral, the mouse will not appreciate your neutrality.” Bishop Desmond Tutu
Basically, if you make 11 Siri queries a day every day for a month, you're going to use about 20 MB of data in total for the month.. Not much at all... I think your friend has used up their data doing something other than asking Siri questions..