How long does shoulder tendonitis usually last?

Variable: So-called shoulder tendonitis can actually be one of a few things and specifically tendonitis of the biceps tendon or possibly bursitis, arthritis, rotator cuff pathology etc.. Initial treatment is usually but not always rest along with nsaids and hot/cold and massage. No matter the exact pathology it's important to keep the shoulder moving at least a bit to avoid a frozen shoulder.

Days weeks or months: It depends how bad it is. It can improve with nsaids, rest, ice , gentle range of motion, cortisone injections, physical therapy and last resort surgery with an arthroscope.