How much data does Siri use in a month? Well, that depends on how many instances you use Siri per day, according to tests by Arstechnica. They performed a variety of tasks for Siri that required local tasks and those that needed lookups online.
In the end, they determined Siri isn’t that much of a data hog, based on moderate usage:
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.