The best thing you can do is try to limit the amount of walking you do on that foot, this avoids more tissue damage to an already damaged area. If you have to walk on it then make sure you wear well cushioned, flat shoes so that the heel has plenty of protection. You could also try rubbing a little lavender oil on the bruised area, either straight out of the bottle or mixed as a lotion with a cream. Lavender is good for helping to reduce bruising.
As for how long it will take to heal really depends on you as a person and how badly the soft tissue is damaged. It could be days or weeks, but hopefully not to long.