When to have cataract surgery is your decision. Yes, yours.
A cataract is not like fruit, as fruit can be overripe and spoil, but instead like fruit in that one decides to eat it based on personal preference. The decision is based upon your tolerance for decreased vision and your doctor's approval that surgery would be helpful.
There are instances, such as the presence of diabetic retinopathy or macular degeneration, where other factors influence the timing of cataract surgery, but removal of cataracts is usually an "elective" procedure, in that, there is rarely any true medical emergency.
Don't worry- most cataract surgery is covered by your insurance. The usual criteria for considering cataract surgery is simply how well you see. If the you do not see well or adequately, despite a change in contact lenses or glasses, you may consider surgery. If, on the other hand, you are pretty happy with your level of vision and function, then maybe you can wait. Obviously, consult your eye doctor to make the ultimate decision.
Sometimes the vision at night is the only problem. Commonly, people experience tremendous glare with approaching headlights. Glare testing can be done at your doctor's office. Patients with macular degeneration or diabetic retinopathy may have additional considerations. Patients with diabetic retinopathy should have surgery when the retinopathy is stable. The best, and only way, to determine stability is to have your eyes dilated and examined. Usually "macular edema" should be controlled before attempted surgery. Again, your doctor should make this decision.
By definition, patients with macular degeneration have already lost some vision from the retinal disease. Cataract surgery has never been shown to make the macular degeneration worse, but there should be some realistic expectations regarding your vision after surgery. It is important to realize before surgery, that the blurry vision is a result of cataract and retinal degeneration. Cataract surgery will only improve the visual loss from the cataract, not the macular degeneration.
Lastly, the quality of your vision can NOT be measured.