How long does the average juvenile rheumatoid arthritis flare up usually last?

Each different. There are a variety of differ scenarios. It depends on the type of activity, the type of juvenile arthritis, and the type of therapy.
Must treat early. Much depends upon the medicines being used and how quickly medical treatment is added after a flare starts. Often the addition of low dose steroids can instantly make a difference while other disease modifying drugs are used or changed. Check this link: http://www.Arthritistoday.Org/community/expert-q--a/rheumatoid-arthritis/flare-last.Php.