Endurance scales, very nearly, by a factor of ( 1 + 3 * [ -1 + sqrt( M / ( M + m )) ] ) where big M is the original takeoff mass and small m is the (signed) mass either added or removed. For an aircraft similar to a C172 this is roughly equivalent to a 15 per cent increase in endurance for each adult passenger removed.
Edit: changed the approximation sqrt(M/(M+m)) to (1+3*[-1 + sqrt(M/(M+m))]. The correct but more complicated factor will be evident from inspecting the Breguet endurance formula.
Last edited by selfin; 28th Feb 2019 at 00:46.