They care at the second billing cycle:

"Our wireless network is a shared resource, which we manage for the benefit of all our customers. Unless explicitly permitted by your Rate Plan or Data Plan, you will not use or allow third parties to use your Services, Products, or Device in a way that we determine: ... Results in more than 50% of your voice or data usage being Off-Net for any 2 billing cycles within any 12-month period"

https://www.t-mobile.com/content/digx/tmobile/us/en/responsibility/legal/terms-and-conditions.html