Final answer:
Allow the APU to spill down for at least 5 minutes before applying halon.
Step-by-step explanation:
When it comes to spilling down the APU (Auxiliary Power Unit) before applying halon, the general rule of thumb is to allow enough time for the APU to stabilize and for the dangerous gases to dissipate. The correct amount of time can vary depending on the specific situation, but typically, a minimum of 5 minutes is recommended before applying halon. This allows for any remaining flammable or toxic gases to disperse, reducing the risk of an explosion or fire.