Step 1: Categorizing Transplant Rejection.
Rejection manifests in three primary forms: hyperacute, acute, and chronic.
- Hyperacute rejection: Immediate onset (minutes to hours).
- Acute rejection: Onset within days to weeks.
- Chronic rejection: Gradual development over months to years.
Step 2: Mechanism of Chronic Rejection.
Chronic rejection is characterized by a gradual decline in graft function. This occurs through persistent immune-mediated damage, leading to fibrosis, thickened blood vessels, and reduced blood flow (ischemia). It is a protracted deterioration process.
Step 3: Selecting the Appropriate Answer.
As the query focuses on chronic rejection, the relevant timeframe is months to years.
Final Answer:
\[
\boxed{\text{Months or years}}
\]