Yes, indeed, you could have had diabetes for a consider-able period of time, months or even years, and been unaware of it. However, it is unlikely that you could have had severe diabetes with very high blood sugars for a long time without having to seek medical attention, as you would have experienced complications.
However, milder degrees of diabetes are often without obvious symptoms, although in retrospect patients will realize that all was not well when they start to feel the benefits of treatment. Studies have shown that, on average, type 2 diabetes has been present for several years by the time it is diagnosed. It is important that asymptomatic diabetes is detected and treated, because it can lead to serious health consequences, which may be irreversible when detected.
About one of every three people has detectable neuropathy (nerve damage) at the time of diagnosis, indicating that longstanding diabetes has been present. Less commonly, eye damage (retinopathy) and/or kidney damage (nephropathy) are discovered at the time of diagnosis. These are serious consequences of diabetes and are the leading causes of blindness and kidney failure in working age adults in the United States, as well as much of the industrialized world.
As serious, or even more serious than this, is the potential for undetected and untreated diabetes to lead to heart attacks, strokes, heart failure, or amputations. Indeed, almost 4 of every 10 patients admitted to hospital with a sudden serious cardiac event will be found to have previously unsuspected and undiagnosed diabetes or prediabetes. It is now clear that prediabetes is almost as serious a risk factor for such cardiac events and death from them as full-blown diabetes.