According to an investigation done by Ars Technica, Siri uses much less data than many users feared. Each query is sent as an audio file to Apple’s servers, which then return the results back to the iPhone. The average amount of bandwidth used per query is just 63KB, so even if you use the voice assistant over 3G a few times a day for a month straight, you’re unlikely to eat a significant chunk of you’re monthly quota. Of course, after the honeymoon phase is over, we’re sure Siri usage will drop considerably, making the data consumption completely negligible.
If you use Siri 2-3 times per day at an average of 63KB per instance, you might expect to use 126KB to 189KB per day, or 3.7 to 5.5MB per month. For 4-6 times a day, that might come out to 252KB to 378KB per day, or 7.4 to 11MB per month. If you use it 10-15 times per day, you might end up using 630KB to 945KB per day, or 18.5 to 27.7MB per month.