If appr acitaminophen dose is given to infant WRT age/weight then how long this treatment can safely (w/o toxic effects) cont. Is ten days too much?

Shorter is better. In infants, cause of fever is generally infections, which shouldn't take more than 3-5 days to subside with the recommended treatment for the condition. Due to his age, he should be seen by his pediatrician. Important thing is to make sure he remains well hydrated.