By default, your Google Stadia games stream at the best visual quality possible depending on your device and membership tier. For anyone worried about data caps because their internet service provider limits how much data can be used in a month, this isn't ideal. There's a quick and easy way to limit your data usage so long as you're okay with limiting your streaming resolution as well.
This will drop your streaming quality down to either 720p or 1080p depending on which you choose. Data usage can reach 4.5 GB/hour at 720p while streaming at 1080p could use up to 12.6 GB/hour according to Google. Only those who subscribe to Stadia Pro have the option of streaming in 4K.
Alternatively, if you have a strict data cap and already stream a lot of content through services like Netflix, you may want to forego game streaming entirely in favor of downloading your games on a PC or console.