"...on average how many years after diagnosis do complications begin in cases like this?"
Considering there has been 12-13 years of negligence, diabetes complications began years ago. Some may be more noticeable than others such as fatigue, thirst, and frequent urination. Others might include blurred vision, poor wound healing (cuts, scrapes, etc.) and/or recurrent infections, dry mouth, and dry or itchy skin. Serious complications are retinopathy and neuropathy in hands and feet.