Re:fibula spiral fracture at ankle: additionally, I have had no pain since break, and walking lightly since cast was put on, question is about a boot?

Cast now boot later. Is this being treated without surgery (ie a nondisplaced break)? Anyhow, if so usually is 6 weeks immobilized and non weightbearing to allow for healing and minimize the risk of displacement. I tend to not use the boot until after this initial 6 week period. For a very hairline type break i might allow a boot as long as i can trust the patient to keep it on near full time.
Talk to your Dr. It is best to talk to your doctor since they know the exact injury and stability of the fracture.