How long does HGH stay in your system? Had one injection of omnitrope somatropin 5.8 mg into my back a couple days ago.

Hard to say. Since that's a fairly large dose, probably hours to 1-2 days at most. Why would you take such a large one time dose? Have you been diagnosed with hgh deficiency or are you taking it for weight loss/muscle building/life enhancement purposes. If it's the latter group, such administration is not only illegal, but also medically dangerous and should be avoided.