Usage will all depend on what EO you're using. Citrus behave differently from spices, etc. There are usage calculators like Brambleberry's that may give you a rough guideline for certain individual FOs and EOs they sell. Other EO sites may have similar guidelines. Generally, you may use less EO than FO. But testing is the only way to go.
Yes, check safe usage rates. I use Brambleberry's calculator.
As to how long they stick - it depends. Patchouli sticks very well! But it's pretty expensive for soap, IMO. Citrus EOs fade fast, though citrus FOs aren't much better. Lemongrass and lit sea cubeba stick better and are "citrusy" but not true citrus.