Cell life-time is variable from hours and minutes to many days or years. Each cell-type can exist independent of any other:

• "All living things consist of one or more cells."
• "Each cell can live independently of the rest."
• "Cells can arise only from other cells."

Are Alzheimer and dementia issues genetically and differently time-programmed for an individual so that brain cells regenerate and the associated memories contained within a particular neuron cluster can be lost as new cells regenerate? Most Alzheimer and dementia sufferers are older individuals, but this is not exclusive. This suggests that such electronic information is long-term temporary and different individuals have different spans of memory retention. Short-term memory is related to damaged or impaired brain tissue and can be caused by lesions (tumours). Not necessarily the lesion itself, but the secondary effects of the lesion. Tissue compression by a pressure of a growing mass. A tumour is a brain cancer by association to an uncontrolled growth.

Life span for different types of blood cells:
red blood cells are 120 days
platelets are 7-12 days
neutrophils are on the time scale of hours/days

If you take the longest time of 120 days and assume all other blood cell types are being replaced at a much quicker rate, then human blood is "regenerated" every 120 days.

The rates for different cell types are highly variable. There are many parts of the body with extremely slow rates of turnover such as the heart and the nervous system. Some researchers would even suggest that for practical purposes there is no regeneration in those tissues.