If everything is working as expected (no black bleed, IPD feels good, no juddering, etc) then I can play as long as I want in Elite. Last weekend I had some rare downtime where I wasn't needed anywhere, so I slapped on the Rift around 8 PM and didn't stop until I felt a bit tired. When I pulled the Rift off it was 2:30 AM.
Prior to the Rift I had (and still have, just disabled) triple monitors on nVidia Surround with TrackIR. That's nice, and it's still great for showing someone how nice the graphics are in the game, but once I put on the Rift I could tell there was no going back; screen door and difficult to read text included. The immersion is excellent, and the scale is impressive. You just feel like you're there, right there, look down and see your body, try to reach out and touch things. I find it annoys me when the in-game hands fidget and I'm not actually doing that; it breaks the brain a bit to watch you do something you're not really doing.
Now, put me in Alien Isolation and it's a completely different story. I can play about 45-60 minutes before taking a break. I never get motion sick, but that game will give me cold shivers and feel like I need to stop the world from spinning. I've heard ginger helps with this, or motion sickness pills, so I may try that to see if I can overcome. An hour of play and then a break might sound good, but it generally takes me 2 hours or more to recover once I feel sick in A:I.