After a tibia and fibia break how long it will take me to regain muscle in my leg?

Muscle. It depends on how much has been lost and the time it took to heal . Your surgeon can refer you to physical therapy for the help you seek.
Same as time in cast. It will take about as long to regain strength as your leg was immobilized, assuming proper strength training. Consult a physical therapist for help unless you already know a lot about muscle training.