Rather than there being a time limit under which the components of the feedback loop must act, I meant that there are some seasons of the year when El Nino (or La Nina) is harder to initiate, such as during late fall or winter, and other seasons when the normal climatology of the tropical Pacific Ocean is more conducive to getting the feedback going. It is easier from mid-spring (northern) to summer, when the SST is normally at its wamest as a starting point.