As long as they want, assuming they don't take on the accounting practices of the bigger carriers. Companies like Verizon and AT&T make a ridiculous profit on their data service, yet they cry over losing even one cent. Here's my math to back it up.
I was bored one day and decided to do some digging on those data overages the cell phone carriers love to charge. $.10/MB seems like a fairly common rate, but what people don't realize is, that works out to about $33,000 per Mbps per month. The amount of data you can send in one month at a rate of 1Mbps is about 330GB, or 330,000MB. At the overage rate of $.10/MB, that's $33,000 a month. That's a new BMW, every damn month.
I know what you're thinking, though. "Well, it's a penalty. It's supposed to be ridiculously high..." Yeah, that's understandable, but it's not much higher than their normal data rates. Taking Verizon's plans as an example, 1GB is $50/month. 50,000 cents divided by 1024MB is $.48/MB. But that's unfair because that includes unlimited minutes and text messages, so let's consider this - their 2GB plan is $10 more. That's $10/GB. 10000 cents divided by 1024MB is $.094/MB.
9.4 cents per MB regular rate.
10 cents per MB at overage rates.
Either way, we're paying $33,000/Mbps for cellular data service. Do you know what the going rate is for IP transit in the telco industry? At bulk rates, $1-2/Mbps.
tl;dr: T-Mobile can play the Unlimited Data game as long as they want. The cost to them is low, they just choose not to fuck their customers as hard as the big guys do.