How long will i need a cast for a talus fracture?

It depends: Was your talus fracture fixed surgically? What type of talus fracture was it? If you had surgery for your fracture, i would keep you casted for 4-6 weeks, and then advance you to a cam boot (looks like a ski boot). The key is not the type of immobilization but non-weight bearing, as putting weight on the foot too soon could lead to displacement or non-union of the fracture.

Depends...: 6-8 weeks on average but depending on the nature of your injury sometimes longer.

At least 4 weeks: Provided fracture is stable. You can transition to a fracture boot after the cast. You will need to continue non weightbearing for at least another 4 weeks to allow for fracture healing.