The concept of seismic gap has been developed in recent years to identify areas which have not been active in the past few years and are, therefore, considered to have potential for large earthquakes in the future. I propose an empirical method to determine the duration of quiescence in seismic gap by estimating slip which will occur when the gap ruptures. This method depends on rate of plate motion, length of seismic gap, and empirical relationship between rupture length and seismic moment. For earthquakes which have occured in previously identified gaps and for which duration of quiescence is known, this method predicts quiescence time to within about 10 years, which, considering errors in plate motion rates, length of seismic gap, and empirical relationship involved, is very good.