So it doesn't depend on time of day, but how many other customers are sharing your tower.
https://www.t-mobile.com/customer/mydatausage.html
> Data prioritization will only be noticeable when you access a congested tower and have used over 50GB of data in a particular billing cycle. You will continue to get unlimited high speed data on your smartphone when you aren’t accessing a congested tower. In that way it’s different from data throttling, which slows you down for the remainder of your bill cycle regardless of network conditions, all the time.