If someone has hand, foot and mouth disease, how long will they be contagious?

Up to 1 month. The disease is most contagious during the first week of illness, but the virus may spread after the signs and symptoms have disappeared because it persists in fecal material for up to 1 month. [cdc: hand, foot, and mouth disease].
Depends on your view. The hfm viral illness is caused by viruses that will be excreted in the stool for some time (wks) after primary infection.The febrile period (days) they may pass the virus in oral secretions. The texas dept of health standard does not exclude these kids from daycare after the fever is gone. Standard hygenic practices during diaper changes will contain the spread from that source.