Your consultant should instruct you when you can start physiotherapy. Your physiotherapist can then tell you when to start full loading of the leg and when to start gym exercise. There is no golden rule. Just to give you an idea. The Fibula is one of the fastest healing bones. Normally you could start with weight bearing after 1.5-2 months after immobilization. You have definitely a delayed healing therefor any speculations are futile, the reasons for this delay can be numerous.