greenfrog,

If you are using your computer and find that you have been staring at the screen or rereading a line over again or your eyes start to get glassy, that is your limit. Next time, change tasks after a shorter time period or at the first sign of spacing out. With a little self awareness, you will start to notice the overdoing it before it causes symptoms that set you back.

It is different for everybody so you have to find your own limits.