I'm a heavy smoker with an ankle fracture. It's been 6 weeks now with cast and doc advised another 2 weeks, it's undisplaced so when can I walk fully?

When healed. It should be healed on X-ray, but also non tender before you should begin walking.
Varies. Broken bones definitely heal more slowly in smokers, thought to be secondary to the vasoconstrictive properties of nicotine. It also depends in part in the more specific nature of the fracture. A non displaced fibular fracture can be treated with immediate weight bearing, but a non displaced, interarticular tibial fracture needs to be healed before you walk on it.