How long should one use prednisone for?

As short as possible: Prednisone is a miracle med that puts many problems in remission but comes at a price of : anxiety, depression, agitation, increased risk of infection and bone fracture. Long term use requires a serious discussion of relative risk and benefits.

Shortest possible: The longer you take it the more risk for long term effects: osteoporosis (bones that fracture from lost calcium), early cataracts. If you must take it you must but know the risk.

It depends: It depends on the reason it was prescribed in the first place. As a rule use Prednisone in as low a dose as possible for the least amount of time as possible.