It's a complex topic, depends on age, genetics, corrective lens or not, nature of your work and neuroplasticity. However, the eyecare industry agrees that max period for continuous closeup work (screen, paper, objects) is 20 minutes.

https://opto.ca/health-library/the-20-20-20-rule

2018 thread: https://news.ycombinator.com/item?id=16146106

My parents always said I'd get square eyes if I watched TV too much