New Grower (outdoor grow)

I really didn't plan on growing, but I discovered a single seed left on my garage work bench. Two weeks before finding the seed, I had planted a couple of tomato plants in pots in my back yard. So I put the seed in the pot with one of the tomato plants. After it sprouted, I decided to do some research and learn something about raising the plant (I knew absolutely nothing about growing). So after learning that it was female plants the produce the "good stuff" I just let it grow until I could determine the sex. Once I decided it was female I removed the tomato plant from the pot. I think it's a sativa, but not really sure. I'm just looking for some advice as to when I should harvest. I live in Indiana, I first noticed the seed had sprouted on 5/12/17. Any advice is greatly appreciated.