Definitely length of day, not temperature. Cloudy days speed it up.
That being said, over time, natural selection will select for denser fur in areas that are historically colder like far north and high elevation.
So, primeness is dependent on daylight. Fur density is dependent on genetics and nutrition (don't forget that). Just like people with poor nutrition get bad hair and nails, fur is worse with poor nutrition.
When do NV cats get prime is a tough question. If you grade by condition of leather (blue vs. not), then they are prime now. If you go by fur length, then everyday until April they get better.