I have a friend once got red eyes and her recover section last so long then I accompanied her to the hospital, the doctor told us the redness is a sign of irritation. This can be due to dry eyes, allergies, infection, or inflammation, and can range from mild to severe.
In addition, thyroid disease can also be linked to dryness, and chronic red eyes. Other more rare causes would include some of the autoimmune diseases such as Lupus or Rheumatoid Disease.
I suggest that you take blood tests to confirm these problems, and sometimes treatment of the entire body is needed, with the red eyes resolving at the same time.
The reason for you may be dry eye, blepharitis or allergies, but if you have concerns, or if eye drops are failing to help, just go to see a doctor.