Yes, some of them can appear immediately after the shock (if there was one).
I like using the example of a cut in your skin as an analogy to the injury suffered that ends up as PTSD. I find it useful to explain it that way.
When you suffer a cut, you are injured; you bleed, the area is swollen, you have either pain or numbness, etc. Then, your body starts working really hard in closing the wound, and you’ll go through a process of healing unless something happens that makes the wound worse, like an infection, or if it opens again before it’s closed and scared.
With PTSD something similar happens. When you experience an event that you subjectively consider dangerous, your nervous system suffers an injury. That injury is caused not by the event, but by your brain and body. The knife that cuts your skin is not the injury; the event that frightened you is not the injury either; the injury is the way your system responds to the threat.
Your autonomic nervous system activates a series of processes that flood your bloodstream with stress hormones, and a whole set of changes happen in your body because of that.
Your skin opens because it was not able to protect itself from the knife. Your system gets injured because your emotional system gets activated and opens. Fear is the sharpness of the knife. You bleed the same way that your adrenal glands secrete cortisol and adrenaline.
The same way that your skin swollen, your legs and arms get stronger to be able to fight or flee.
Your skin may feel pain the same way that the heart increases its rate to pump more blood into some areas of your body to make them stronger.
If the cut is deep, your skin will be numb to avoid more bleeding and pain, and in the same way, if the threat is serious, your consciousness will be numb to avoid pain and suffering, you’ll forget, you’ll dissociate.
To answer your question, you know that in the case of the cut, there is a sequence and a process. Some “symptoms” of the cut happen immediately, and some a little later.
Bleeding is almost immediate, but numbing may take a few minutes. Bleeding will stop eventually, but either the pain or the numbness will stay longer.
At some point, if there is no extra cutting or external aggressors, the wound will close and heal. In PTSD, if the threat is gone, and there is no more fear, the injury will subside and the system will “close” back to baseline.
If there are other problems, like an infection, which for PTSD would be more stress or extra emotional load (like shame), then the symptoms can get aggravated and healing could take longer.
In the same way that the cut can develop an ulcer, the nervous system can develop a chronic state of survival that would perpetuate the reaction after the injury.
If there are more cuts, then the wound gets deeper and following the analogy, the deep cuts or the multiple cuts would become complex trauma (C-PTSD).