I’ll wager that monastics primarily used an incense clock, or oil lamps & candles (…almsbowls as water clocks?). But otherwise the only information I can find involves Vedic time cycles, from Brahma’s lifespan to the time of one eyeblink.
Wiki suggests that a Yāma = 1/4 of a day (light) or night, so eight Yāmas make half of the day (either day or night).
Here’s something about praharas (“The day is divided into eight parts: four praharas for the day, and four for the night”), in a footnote:
Some scholars correctly infer that in seasons (and regions) where days and nights are unequal in length, the praharas expand and contract in length. See Duncan Forbes’ early comment on the pahar (= prahar) in Northern India: “The first pahar of the day began at sunrise, and of the night at sunset; and since the time from sunrise to noon made exactly two pahars, it follows that in the north of India the pahar must have varied from three and a-half hours about the summer solstice, to two and a-half in winter, the pahars of the night varying inversely.” (Duncan Forbes, LL.D, transl. Bāgh O Bahār; or Tales of the Four Derwishes, by Mīr Amman of Dihli. London: W. H. Allen & Co. 1882, (p. 23, note 1). What we are encountering here is a difference between modern cultures that rely on clock time, and traditional cultures where the length of a day is observed in the sky, from sunset to sundown.