In trial, we use one complete drop on a cotton ball or q tip. In training however, I recommend using more. First, it's because we don't let it "sit" as long in training as it does in trial, second, you want to start with a strong difference in source, versus fringe. More scent in the beginning will tie the reward in with source much better. If you train with too little/too faint scent, when you get to a trial, the dog will hit on fringe odor.
Make sure that you "contain" the scent in training, as well. I use small plastic "paint pots" from the craft store. They are just big enough to get a full cotton ball in, and have a nice plastic snap cap that you can poke a hole in with a pocket knife.
Let me know if you have any other questions!