I had taken multiple times 2-3 weeks long vacations, break from work. Would still use laptop/phone, but my usage was very low, maybe an hour every few days. I still felt the instant discomfort when using screens, but those weeks did not have any headaches nor eye strain.
Once I switched job and took 2.5m gap. When I started the new job after that long break I still felt instantly light discomfort but it was not as bad as usual, there was definitely improvement, but ~2 weeks into the job it all went back to the old bad days.