Most strains need about 12hrs of darkness to fully flower but preflowers are different. They're a symptom, while full-on flowering is a condition that displays the preflowers symptom.
Our vegging plants were recently K deficient because we had to replace our primary Potassium source. When we finally got a fresh bag of organic 0-0-30 a couple weeks ago and gave the hungry vegging plants an extra dose of K, two days later every one had a couple small preflowers growing at the nodes. They have been in 24/0 since rooting.
In most plants, unhindered flower production requires about 12hrs of darkness but preflowers can occasionally pop up even when the plant is not in flowering mode, and doesn't necessarily mean it has entered that state, either.