Half a teaspoon or so is usually sufficient. It should smell pretty strong in your jar. You want things to die pretty quickly, so if things are starting to take longer, add some more. It’s not an exact thing – you just sort of judge for yourself as you use your jar.
The Dragonfly Woman 
Exactly how much ethyl acetate do you pour in?
Do you saturate the dried and hardened plaster of paris, just put in a few drops, or what?
Half a teaspoon or so is usually sufficient. It should smell pretty strong in your jar. You want things to die pretty quickly, so if things are starting to take longer, add some more. It’s not an exact thing – you just sort of judge for yourself as you use your jar.