Siri tasks use varying amounts of data, and have been categorized into 11 different query types in terms of data usage. Internet lookups require between 23KB and 187KB data, depending on the question at hand, or an average of 63KB per query. Here are the results from the Ars Technica study:
If you own an iPhone 4S and perform all 11 of these same queries every single day for a month over your carrier’s 3G connection, you can expect to use roughly 20MB or so in a 30-day month. 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.