After a hard impact (martial arts) my shin bone seems to have been chipped. the area is now all mushy, how long before it is replaced with hard bone?

Contusion: It is more likely that you have a hematoma (collection of blood after an injury). The shin area is very commonly involved since there is a small amount of flesh between the skin and the bone. It will resolve in a month or 2. If you think you have a bone chip get it x-rayed .

This may not be a : Bone issue but a hematoma ( blood under the skin) or fat and/ or muscle necrosis (dead tissue) that feels mushy-see a physician for evaluation to see what the diagnosis is before assuming it is a bone issue which may also be bruised or contused.