The interest rate for both the HIBOR (Hong Kong Interbank Offered Rate) plan and prime plan changes over time. However, the HIBOR changes more often while the prime rate is more stable, so prime plans are more predictable. Take note that there are two prime rates that banks in Hong Kong quote: the high prime rate (5.25%) and the low prime rate (5.0%). These rates have held steady since 2008.
HIBOR, on the other hand changes more often, so as HIBOR-based plans. HIBOR plans are renewed periodically (monthly or bi-weekly) during the loan's tenor and you can choose to set your interest period at 3, 6, or 12 months based on HIBOR when you apply for the loan. If you have chosen a 3-month period, the interest rate will be revised every three months. Generally, the longer the interest period, the higher HIBOR will be.