How often the water needs to be changed depends on how often you drive. Our semi truck drivers should change their solution every 3 months. A passenger vehicle twice a year unless you are driving constantly. In the interim you are only replenishing the reservoir tank with distilled water as needed.
You can safely pour the KOH (potassium hydroxide) solution down the drain or toilet. Same stuff that "Draino" is made from, but do not use a unclogging plumbing chemical, it has added ingredients.