How many days does a 节气 last for?
before 清 dynasty (1644-1911), the calendar system in china used 平氣, solar terms lasts 15 days+ (it's roughly 365.25 days / 24).

then, the 時憲曆 changed the method to 定氣, that 360° div 24 = 15°, upon people observation, when the sun "walks" 15° on zodiac, solar term changes.

so, your question would be answered, it depends on which dynasty, and / or which seasons. eg, in 2018:

the 夏至 (summer solstice) is on 21st june, the 小暑 (lesser heat) is on 7th july. so, the summer solstice lasts 16 days :)

http://www.weather.gov.hk/gts/astron2018/Solar_Term_2018_uc.htm

have fun :)