How can I tell how much time I've spent in an Xbox One game?
Starting with the February 2015 software update (which came out after I wrote the question and accepted TZHX's answer), this information is available on the console itself.
As of April 2019, here's what you do:
- Highlight the game's icon on the Home screen or My Games And Apps
- Open the app options menu, such as by using the controller's hamburger Menu button
- Select Go To Official Club
- On the new screen, change from the Feed tab at top to Progress by scrolling left or right
- Change from the Achievements sub-tab at left to Stats by scrolling down to the lower area, then left to the sub-tab list, then down
From here, you can see total time played as well as other game-specific statistics, and you can also choose to Compare With Friends.
Strangely, it doesn't seem like this information is available on the console itself.
I say strangely, because it is tracked and available to view under the "Achievements" page in the SmartGlass Companion application for tablets (at least, it is for the iOS version). But it doesn't appear to be visible anywhere on the console.