My 3year old has a viral infection, sick since sunday. how long can this last, and when should we worry?

Varies considerably: Duration of illness will vary with the germ involved, the target tissues and past experience with a similar germ. A ten year old with a dozen past illnesses with a cousin germ may have cross immunity that limits a virus to a day or two. A 2 yo with a first experience with the same virus may require a week more to mount an immune response that ends it. If the kid stays hydrated & plays some=good.