I believe that nightmares are the most common symptom of PTSD, especially severe trauma. They can last for years, even decades in the case of war trauma (many veterans of the American war in Viet Nam continue to have nightmares). However, I have not found nightmares to be predictive of anything about the condition.
Since nightmares might be part of the definition of PTSD, if you researched this (or have), think you'd want to be sure to remove nightmares to be able to assess this as a potential predictor of PTSD. There was an abstract published in SLEEP (Tammana et al) - that found that CPAP reduced nightmares in patients with PTSD and apnea. https://www.sciencedaily.com/releases/2013/07/130708143317.htm