Duration of load is a design factor that is applied to the allowable stresses for a structural member. That factor typically ranges from 0.9 to 1.6 and is based on the type of loading the structure is expected to resist. Design loads are classified based on the cumulative amount of time (50 years 'permanent' to 10 minutes 'wind/seismic') and they are assumed to act over the life of the structure. The shorter the time period, the higher the factor. Additional information can be found in the National Design Specification of Wood Construction®, ANSI/AF&PA, NDS-2012, October 2012, American Forest & Paper Association.