What is duration of tithi according to Hindu panchang?
From Vedic Astrology: An Integrated Approach by P.V.R Narsimha Rao
In lunar calendar, one day stands for one tithi. Tithi or lunar day is a period in which the difference between the longitudes of Moon and Sun changes by exactly 12°.
Moon and Sun longitude positions are important to determine the tithi and when it will change.
When Sun and Moon are at the same longitude, a new lunar month of 30 tithis starts. As time progresses, Moon will go ahead of Sun. When Moon’s longitude is exactly 12° greater than Sun’s longitude, the first tithi or lunar day finishes and the second tithi starts. When Moon’s longitude is exactly 24° greater than Sun’s longitude, the second tithi finishes and the third tithi starts. When Moon’s longitude is exactly 36° greater than Sun’s longitude, the third tithi finishes and the fourth tithi starts. And so on. You can see that Sun-Moon longitude differential will be (12 x n)° after exactly n tithis.