When does the timing belt need to be replaced on a 2004 Volkswagen Pissat

Unless you have 100% proof that the timing belt was changed recently (within the last 20K miles), plan on changing it ASAP. Modern VW engines are "interference" designs. This means, if the timing belt or chain breaks, any open valves in the cylinder head will impact any pistons as they continue to travel in the cylinders. At minimum, this will bend the valve stems and require A LOT of cylinder head work. At worst, the pistons will be damaged as well, and that means you're buying an engine. In the long run, religious adherence to scheduled timing belt maintenance will save you a ton of money and stress.