How long do cases of brucellosis usually last?

Depends. If caught early in the first few months of infection and effectively treated, brucellosis can be cured and the risk of relapse or chronic infection is low. But, people can also develop chronic brucellosis and depending on the level of organ involvement may have life-long sequelae despite treatment.
Depends. They can smolder for months, Undiagnosed, they can kill you while you're being treated for something else. Especially, it can get in the bone or on the heart valve and stay there for a long, long time before a diagnosis is made. Treated, it's gone in a few days.