At risk for developing diabetic retinopathy. After how many years could it appear?

Unpredictable. Diabetes is the leading cause of new blindness in working-age adults (age 20-74) diabetic retinopathy (diabetes eye disease) does not have symptoms until the late stages. Diabetic retinopathy is treated with glucose control by the patient and primary care provider and lasers to retina by a physician (ophthalmologist). Unpredictable, so yearly dilated exams are required.
Unsure. No one can tell you that. It may be a few years, it may be many years, it may be never. If you control your diabetes, blood pressure, cholesterol, and weight, you drastically reduce that risk - so focus on that, not when you might develop retinopathy. As in most things in life, if you focus on the negative, the negative is bound to happen.