You've noticed and felt it yourself that your eyes often hurt when you play for hours. My question is, why don't you stop immediately? Don't you realize that it's because you're staring at the screen for too long? Honestly, I'm a slot player too. Sometimes I play for hours, but I've never experienced the same thing as you, especially with your eyes. What I often experience is boredom when the game goes on for too long, my balance just keeps spinning, stagnant.
I don't think there's any better advice than to stop, which is why a gambler should have a time limit when playing.