The T-Mobile Samsung Galaxy S9 is getting an update to target camera stability for users, though it's also containing a stability improvement for the Face Unlock feature as well. The software update, which appears to be rolling out now to some T-Mobile customers who already have their Galaxy S9 devices (which began arriving on doorsteps as of yesterday for those who pre-ordered),only includes these two improvements so it's not going to be a very big update, which means it won't take very long to download and it won't take up a whole lot of space on the phone's internal memory.
More to the point it won't eat up a lot of data should you be downloading the update through the mobile carrier network instead of over Wi-Fi. That said it still weighs in at 259MB, though this isn't really that much bigger than some of the games which you can find in the Play Store. Since this looks like it has just started to hit devices it may still be a couple of days before it reaches all T-Mobile customers who have their phones at this point. When the software update appears, you'll have the option to either install overnight or install immediately, so you can finalize things right away or put it on hold for later if you're currently busy and don't have the time to update your device.
According to some comments on Reddit, some customers had received the update only an hour after getting the new phone which means T-Mobile likely had the update ready to go out the door before the handsets even started arriving for those who pre-ordered them. It's also being reported that Galaxy S9 Plus owners on T-Mobile are getting the update as well so it's not just limited to those who picked up the smaller Galaxy S9. The update does not include the March security patch according to users on Reddit who have already installed it, though it's also clear it isn't included as it would have been described in the information page that pops up when you tap to install the update, a screenshot of which you can see below. If you haven't received the update yet it shouldn't be long before it shows up.