Never. You can't walk on a broken fibula. You have to wait until it is healed, and not broken anymore. :) This can take anywhere from 6-8 weeks, or longer, depending on the severity and type of fracture.
I would say about 6 months or maybe even less.That depends on if you take care of it and not interfere with it.In other words follow some tips the doctor might have given you.
I had a bad accident in November 2008 and broke both fibulas and my right tibia. It is now late April 2009 and while I can certainly walk on both legs at pretty normal speed and ride a stationary...
Six to eight weeks. Expect to limp for a while after that until the muscles build back up. Your doctor should have explained all this to you when you got your cast.