Final answer:
The processing time required to form a permanent wave is affected by temperature, humidity, and hair length.
Step-by-step explanation:
The processing time required to form a permanent wave is affected by all of the above: temperature, humidity, and hair length. Temperature can affect the speed of chemical reactions involved in forming the wave, humidity can affect the absorption and retention of water molecules in the hair, and hair length can affect the amount of time it takes for the chemicals to penetrate and react with the hair shaft.