Full-length cycling pants (often called tights) are your go-to when temperatures drop below about 8°C (46°F). They give you complete coverage from waist to ankle, keeping your muscles warm and protecting your skin from cold wind and damp conditions. This matters especially for your knees, which really feel the chill on long winter rides.
The trade-off is that full-length tights can overheat you in milder weather. They're built for cold-weather riding and winter training—not for those crisp fall days where 3/4 length might be plenty. If you run hot or push hard on the pedals, you may find full-length too much above 10°C.
Look for thermal or fleece-lined fabrics for the coldest days, and make sure the ankle closures fit snugly without restricting circulation.