How often can I get Demon dust by dusting Demon?

Question

I want to make achievement Ashes to Dust which is btainable by using blessed wooden stake 500 times successfully on slain demons and/or slain vampires. Since I exp on Frumbras' way where is many demons I decided to make it. 

My questions are:

• How often can I use Blessed Wodden Steak?
• What is the chance to get a Demon Dust?
• And how much time do I have to use Blessed Wooden Steak on dead body?

Answer 1

The statistics taken from HERE show as follows:

You can use the Blesses Wodden Steak immidiately after the demon has died.

I'm not sure how long does it take for a monster to decay, but you can use it on the body, as long as, it is considered to be a "fresh" corpse.