The fidgeting/not sitting still is typical of chorea - a movement disorder caused by many things one of which is rheumatic fever
Most of the knowledge about the course of sydenhams chorea comes from groups of cases or personal experience, hence the variation in the details of the clinical course of the problem - the time frame of weeks to months is probably correct, but it is hard to predict which end of the curve a child is on, although mild symptoms should mean a shorter duration in general and a less severe problem
MRI may help in prognosis as some reports suggest that there are basal ganglia (the deep parts of the brain controlling movement) abnormalities in more severe cases.
Good luck