If someone has myositis does it stay in one muscle or spreads throughout the entire body? if it does spread how long would that take?

Depends: There are many different tyes of myositis with various causes . Need to be more specific.

Symmetric & Proximal: Myositis is inflammation of muscles caused by autoimmune attack of the muscle tissue. It can sometimes be painful but the primary symptom is progressive weakness (over weeks and months). Affected muscles are usually proximal (closer to the body) such as shoulders and hips while wrists/hands and feet are usually spared. Sometimes accompanied by Raynaud's and purple/red rash around eyes and on chest