is there a period of dopamine loss or disease process going on before symptoms occur?

Question unclear. Dopamine in the brain is associated with several disorders, most clearly parkinson's disease. The question does not state what disorder is the topic of interest. Schizophrenia and adhd also are thought to involved Dopamine systems, but there is no evidence to suggest that either really are a direct result of Dopamine loss, just that medications may work through dopamine.
Parkinson's . I presume you are asking about parkinson's disease, which begins probably about 10-15 yrs prior to clinical signs. It involves loss of Dopamine innervation of the basal ganglia, and especially affects pigmented cells within the pars compacta of the substania nigra. When cell loss exceeds 60%, the deficiency results in clinical motor symptoms.