How fast does Parkinson's disease progress?

Slowly. Once the diagnosis is made, the initial problems can be handled with Dopamine agonists and seem stable for 1-2 yrs, but then the progression requires l-dopa, which may help nicely for about 6-8 yrs, during which time additional meds may be added. By 10-12 yrs the medications are sharply losing efficacy, and the next step may require use of a deep brain stimulator device.