When do people need to get dialysis?

Depends. Dialysis is initiated in advanced stages of kidney disease when one develop symtoms due to toxins accumulating in blood or when excess fluid builds up causing shortness of breath or swelling of thebody. Kidney doc evaluates the symptoms of the patient and correlates with level of kidney function to decide initiation of dialysis.
When kidneys fail. Dialysis is started when the function of the kidneys is inadequate to remove the metabolic byproducts of metabolism. In practice this is usually determined by measuring gfr. Most people have to start dialysis when GFR drops to 10-15 though it may have to be started higher or lower than that.