If a female teenager was to get a heart transplant that was chronically rejected, how long would it take to realize that?

Depends. Any heart transplant can reject acutely or chronically. That is why the best match and candidates are chosen. We have a shortage of donors also as a significant limitation. Chronic rejection can be suspected on clinical grounds and confirmed on periodic biopsy.
Rejection. Transplant rejection is not usually subtle. Most programs test frequently looking for rejection to catch it early, especially in teens. Chronic rejection is not likely to be missed if the patient is following the program.