Jacqui Cheng investigating Siri data usage:
We performed six tasks that would be considered to be local tasks. These queries included things like, “Set an alarm for 3 hours from now,” “Make an appointment for 2pm on Friday” (and then telling Siri to cancel the task), “Remind me to file expense reports when I get home,” and “What is the contact info for Ars Technica?”
These tasks added up to a total of 220KB of data usage, or an average of 36.7KB per query. The actual numbers ranged from 60KB down to 18KB, and we believe this is correlated to the complexity of the specific query and language we used to perform it.
In other words if you are a heavy Siri user (using it 10-15 times a day for such a task) you are looking at adding 30MB of data a month. That’s a lot less data that I expected it to be, well done on Apple’s part.