The last pair I demoed to spent 3 hours in the Rift, with a single person spending two hours without stopping. This was their first experience ever with VR.

VR sickness is a thing and it's varies from person to person and use case to use case, but I have never experienced (personally or vicariously) issues related to eye strain. The lenses are supposed to take care of that by allowing your eyes to focus at a distance.

One of the reasons room scale is making so much noise is that it's a very different experience in terms of VR sickness. Far fewer people get sick in room scale because positional and rotational tracking are good enough that it feels "right". There are fewer motion cues mismatched with visual cues because you are physically moving and the headset is tracking 1:1.

Room scale also has vastly improved immersiveness (now we are calling it presence) especially when combined with tracked controllers and environments that afford all the interactions you would expect from whatever environment you are in.