What would the longest a fever can last in children be?

Depends...: Fever of unknown origin is typically diagnosed after a fever of 4-6 weeks without any discernable cause; however, the grade of fever is an important characteristic. Most febrile illnesses last no more than 1-2 weeks; so you should have the child examined by his/her pediatrician to help discern the cause and prescribe treatment.

Not long: Most viral infections cause fever for 48-72 hours, while bacterial infections can last longer depending on the treatment timing and sensitivity to antibiotics prescribed. In general, if it's been more than a day or so(in children, not infants) it's time to see the pediatrician

Fever- see doc?: When to see a doctor when your child has a fever? Depends on the age and how the child is doing: http://www.parenting.com/health-guide/fever/when-call-doctor