The process of radiative ignition from wild land flames is studied using a simple heat transfer model. For vertical flames (no wind and no slope effects), it is found that the fuel ignition time increases exponentially with distance, which reveals the existence of a characteristic length above which no ignition occurs. The influence of flame size and fuel moisture content on this length is examined. For distances from the flame much smaller than the characteristic length, the ignition time is found to be independent of the moisture content. The existence of such a characteristic length is of major concern for studying percolation-like spread/non spread transition.