T-Mobile's customers have traditionally not experienced any data usage restrictions while roaming on the carrier's partner networks, but it sounds like that'll come to an end this April. According to TmoNews, customers will be capped based on how much "full-speed" data they purchase every month as of April 5th. For example, customers with between 2GB and 4.99GB of full-speed data will be able to use up to 50MB of domestic roaming data each month; once that allotment is up, customers won't be able to use data outside of T-Mobile's network at all until the next billing cycle.
T-Mobile will courteously notify users via text message when they hit 80 percent and 100 percent of their roaming data cap, and a few segments of users (including business accounts) will be exempt form these caps. The carrier plans to start notifying users via bill insert, text message, and email starting in February, so if you're commonly in a roaming area, get ready to adjust your behaviors.