I don't use a preservative. I superfat at an average of 5-6%, and my bars last a long time, the fragrance may dissipate after a certain amount of time (after a year at least if at all). Milk soaps tend to not last nearly as long. I would not count on my soap lasting as long if my superfat was any higher than it is right now.
Lard and palm oil soaps tend to hold up forever. Soy not so much....I use more soy than some so I add ROE as an antioxidant.
The fragrance oil usage depends on the company and the strength of the fragrance. They should have a usage guide posted for soap....they're generally pretty much right. I myself tend toward the low end for heavy spice and florals, and toward the heavy end for citrus.