With the kind of myopic problem we experience in Singapore, its best if we adopt the precautionary approach to eye-care. The national guideline given is for vision rests to be taken after every 30-40mins of reading. If you find it difficult to track your rest periods, just ensure that you look out of your window at the turn of every hour for 5mins.
As you rest your eyes, your mind is given an opportunity to consolidate the information it has just received over the past 55mins. You will come back refreshed and all set to absorb more valuable information!
To find out more, you may wish to have a look at this booklet published by the Health Promotion Board of Singapore http://www.hpb.gov.sg/web/nmpp/pdf/ProtectYrChildrensEyesight.pdf