You cannot overuse your eyes. They may feel uncomfortable or strained but this is from fatigue and your eyes will not be damaged. You may not be able to use them for as long a duration as you once did, but no harm will be experienced from use. Resting and sometimes using lubricant eye drops, as often as you feel the need, will allow you to resume visual activities and your eyes will feel better. Generally endurance improves with persistent practice.
As I know, it will not damage your by using them a lot if you can properly use your eyes. However, if you strain to see a same place for a long time, it will cause eye fatigue. If your eyes start to burn or feel strained, close your eyes for a couple of minutes to rest them.