Commercial Herb Growing in Tennessee

I am interested in growing herbs commerically in Middle Tennessee on 5-8 acres of land. I am not a farmer so I do not have any equipment at present. What can I grow on this amount of land in this location? How much gross would I be looking at?

There are many herbs of commericial importance that you could grow. You can grow herbs used for culinary, aromatic and medicinal purposes, and there are easily 100 or more possibilities.
Because you are just starting to investigate commericial herb growing, we strongly recommend that you read two books: Richard Alan Miller's "The Potential of Herbs as a Cash Crop" and "Herbs For Sale" by Lee Sturdivant. Together these books cover the basics of seeding, planting, cultivation, harvesting and processing, what equipment you need, marketing, prices, potential revenues and much more. Both of these books are available from Richters (see the online catalogue).

What is the growing time for St Johnswort?

The first year harvest of St Johnswort will be very small and there will only be a harvest from plants started indoors early in the year. The second year, you harvest when the first four flowers open and that should happen in early summer.