Here's my aluminum Series 4 which I wore 24/7 outside of charging & showering for almost 3 years. It has the one major scratch (circled in red) that's visible when the screen is on, and it's just barely visible. You can see it best when the screen is off, and I had to work hard to get the right lighting and angle for it to show in the picture. The picture makes it look way worse than it is. There are also micro abrasions, but those are completely invisible when the screen is on, and you have to be looking for them when the screen is off.
I only babied the watch for the first few months until I got that "big gouge" that basically disappears when the screen is on. That, I believe, was caused by a door handle banging into the watch. As you can see from the picture, the aluminum itself is more damaged than the glass from everyday wear (yellow circles).
As far as the environment it was in during those 3 years, I worked in an office but spent limited amounts of time on a manufacturing floor. Outside of work I hike, walk, run, camp, do household chores and some basic car maintenance with the watch on. I also sleep with my watch on, so this one also got banged against a wall and dresser periodically in my sleep. I never did anything too crazy, but I wasn't terribly careful either, and this is the condition it was in after 3 years. It's probably also worth a mention that I ..."tested"... fall detection more than once on pavement and rocky trails during that time. Fall detection works pretty great. And no noticeable damage to the glass from those.
My Series 7 (not pictured) is coming up on 2 years old in a few months and I've been even less careful with it. Same work/life conditions and it's in nearly pristine condition aside from some micro abrasions which are hardly visible even with the screen off. No visible scratches.