The idea that levodopa works for a while and then ceases to be effective is a myth. The myth sticks around because Parkinson's disease is a progressive disorder.
a person starts taking levodopa, the symptoms are nicely controlled. But as the disease worsens, the symptoms emerge again. Because of that, the patient thinks, "Oh, my levodopa's not working," when what's really happening is that the Parkinson's disease is progressing. If you took the Sinemet away at that point, the symptoms would be even more pronounced. But we never do that, so patients don't understand it. And even clinicians who don't treat Parkinson's regularly start thinking, "The medicine has quit working."
Typically, when people develop more Parkinson's symptoms, we adjust their drugs. Depending on the symptoms we're trying to treat, we may increase levodopa or add an additional drug. Although patients usually don't like the idea of taking more than one medicine, this approach may produce a better result -- with fewer side effects -- than larger amounts of a single drug would.