In this work, we often say, "if you know one person with dementia, you know ONE person". Your question doesn't have a cut and dry answer. It is different for everyone, and often depends on other factors like other health conditions such as diabetes and heart disease. Also, some people are diagnosed early in the disease process, and some later. Thank you for asking, because the more we talk about dementia, the more we can raise awareness and decrease stigma. People with dementia can and do live full lives, as those of us supporting them work to understand how to empower strengths and support needs.