Apple's new iPhone 4S has been in users' grubby little hands for a bit over two weeks now. One of the main features of the 4S is the virtual "personal assistant" Siri, a technology that Apple acquired in 2010 and integrated into iOS 5 for integration with the iPhone 4S. When we put Siri through its paces during our iPhone 4S review, we came away with a mixture of bemusement and optimism. The service is still in beta and has its rough patches, but it's pretty decent at performing Web searches and other basic phone tasks, such as setting up appointments, making reminders, and text dictation.

But one huge caveat to Siri is that it requires an active Internet connection in order to function. Apple sends Siri voice data up to its own data centers for processing and the sends the results back down to your phone, even if you're only trying to perform a task that shouldn't require any network connectivity. This can sometimes be frustrating when you're in a low-signal area with no WiFi available, but heavy data users were quick to point out that there are even bigger potential downsides to using Siri: it could force you over your monthly data quota with your cell carrier if you're not careful. And in today's heavily data-capped world, such a concern is becoming more and more valid.

Because of this, we decided to find out how much data a typical Siri user might expect to use in an average month. We carefully watched our data usage while performing 11 different queries through Siri, split between local phone tasks and tasks that required lookups from the Internet. We also performed four dictation tasks. Your milage, of course, will vary depending on the kinds of things you might use Siri for and how often you might do those things. With that said, here's what we found.

The tasks

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.

The five other tasks were ones that required lookups online—some of them were questions that Siri could get from Wolfram Alpha, while others prompted us to continue to search on the Web (thereby bringing up a Google search page with the wording that we had asked Siri). Some sample questions included "How many calories in a muffin?", "How many movies has Kevin Bacon been in?" (this required a Google lookup, which we said yes to), and "What is Lady Gaga's real name?"

These five tasks added up to a total of 473.5KB of data use, or an average of 94.72KB per query. The range went from 23KB to 187KB, depending on the question at hand.

In total, our 11 queries added up to 693.6KB, or an average of 63KB per query. As you can see above, Siri tasks that are local to the phone appear to require less data than ones that need further lookups on the Internet, which makes sense.

In addition to these 11 queries, we also performed four text dictations through Siri—two short e-mails (one was roughly 15 words and the other 40) and two text messages (6-10 words apiece). The text messages came in at 18KB and 12KB for an average of 15KB) and the e-mails came in at 36KB and 109KB, respectively, for an average of 72.5KB. Clearly, the longer the message, the more data will be used, and this will depend entirely upon your own wordiness.

What does it mean?

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. But it's unlikely that that you'll be asking those same questions, or with the same frequency.

I asked around the Ars Orbiting HQ about our average daily Siri usage, in addition to my (and @arstechnica's) followers on Twitter. After filtering out the snarky responses that provided no information, there appears to be (generally) three groups of people who use Siri on a regular basis: those who use it 1-3 times per day, those who use it 4-6 times per day, and those who use it 10-15 times per day. There were a couple people who said they used Siri upwards of 20 times per day, but those were rare compared to the responses I got that fell into one of my three aforementioned ranges. (As an aside, I was personally very surprised to see some of these numbers. My personal usage seems to be in the range of 2-3 times per day, though Ars EIC Ken Fisher's usage averages at about 15 times per day, he tells me.)

Here's where the numbers are going to get rough. As we've already pointed out, your own numbers will be different depending on how wordy you are, what kinds of queries you're making, and how frequently you do it. So please take these estimates with a grain of salt—they're more like general ranges than exact numbers.

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.

The next question is: do you have that much room to skim off the top of your data plan? I pay for the 2GB per month plan from AT&T (which also offers a 200MB tier), and according to AT&T, my average data usage per month usually doesn't go above 250MB. I have plenty of room to add another 4-30MB on top thanks to Siri, but you may or may not, depending on which carrier you're on and what you pay for. Sprint still allows for unlimited 3G data use through the smartphone itself (and not the phone as a hotspot, which is capped at 5GB per month). Verizon offers three tiers for its 3G data: 2GB, 5GB, and 10GB.

So, are the numbers around what you expected, or not? Are they higher or lower than what you thought you'd be spending on Siri? And how much do your own experiences align with what we found?