What are the most obvious symptoms of prednisone withdrawal and how long do they last?

Depends: May depend on the dosage used and the duration of treatment. Stopping abruptly after a long course and high dosage can be a disaster causing life threatening results. More modest dosage may lead to joint pain when the anti-inflamatory effect of the Prednisone is removed.

Fatigue, flare: Flare/resurgence of the underlying disease, fatigue, hyperkalemia, eosinophilia.