At least 24 hours after they are fever free (100 F) (or signs of fever) without the use of fever-reducing medications; UNLESS the person works in a health care setting where the exclusion period should be 7 days from symptom onset or until the resolution of symptoms, whichever is longer. The later, more stringent guideline, may also be considered for people returning to a setting where large numbers of high-risk people may be exposed, such as a camp for children with asthma or a child care facility for children younger than 5 years old.
Source
"CDC Recommendations for the Amount of Time Persons with Influenza-Like Illness Should be Away from Others." http://www.cdc.gov/h1n1flu/guidance/exclusion.htm 5 Aug 2009.