Rosemary extract is suitable as a natural antioxidant in foods and cosmetic/toiletry products. It has a number of more wide-ranging uses which are becoming well documented. Specific inclusion levels for preservative uses are not widely published. It is a natural product and as such it would generally be expected to require higher inclusion levels than synthetics to be effective.
Rosemary antioxidant is a thick, viscous liquid, and due to the colour of the ingredient it may discolour your soaps.