There's no way to tell how long you have had diabetes. When you get the first results and get a diagnosis, the sugars have been raised at least 3-6 months, in some cases much longer. Complications are a tricky indicator since we can develop them in such different rates. Some people's eyes are fine but they may develop neuropathy or periferal vasculopathy and vice versa. The good news is that with tight control some of the damage can be stopped or even reversed.
The question about infections etc triggering diabetes is good, since I think mine was triggered by extreme stress. Depending on the illness, it may affect your pancreas either directly or indirectly and when you add genetics etc into the mix, results may be development of diabetes. But again it is very difficult to confirm any of this thus we're left just guessing in many cases.