As for me, I pay an extra fee to have unlimited data on my private home network. But for anyone with no choice, data utilization is a big factor for Stadia games.
Concerning information use, it will naturally vary according to your quality settings. If you're aiming to play Madden NFL 21 with Stadia Pro at 4K then you are likely looking at around 16-20GB of data usage per hour. As this is a sport you can play solo, online, or locally with friends on the exact same Stadia connection (through Chromecast with multiple Stadia controls or on mobile or a PC browser with multiple controllers plugged in), it's impossible to understand just how much information it will suck since hours will change for everyone.
I played with on Chromecast during my 4K TV, on PC at a Microsoft Edge Browser, also on mobile both with the Razer Kishi and utilizing just touchscreen controls. Unexpectedly, Madden is really very playable using touchscreen controls once you get accustomed to where the virtual buttons are located. I'm still frustrated that I can not remap or alter at which the buttons are on my screen since doing things like holding R2 to sprint are awkward if you would like to also have the ability to cover up the ball prior to an effect or quickly juke with the perfect stick. In regards to the fundamentals of playing though, you can complete an whole game on Stadia using just touchscreen controllers if you wanted or at the very least do some management functions like admin work or go through trade offers.