Prior to the first evidence, or symptoms of Parkinson's disease, how long has the disease been around?
A long while. Parkinsonian movement disorder symptoms generally develop only after about 90% of the neurons in the substantia nigra (a potion of the midbrain). This may require decades to occur. Research imaging techniques can measure the dopamingeric neurons in the brainstem to monitor progress of neuronal loss.
Could be years. The first signs of parkinson's disease can be very subtle and non specific. Memory changes, balance or gait issues, fatigue. They could easily be overlooked or blamed on another process or fleeting illness. The diagnosis is a clinical one with no definitive test to make a diagnosis. Most cases have been around for months to years before a formal diagnosis is made.