Boiling times in recipes vary because many recipe publishers don't know that temperature is the best way to ensure a successful batch. Classic recipes for no-bake cookies have been handed down for generations. Long before culinary science gained steam. So the practice most likely wasn't mentioned in the original written recipe. For best results, track the temperature of the boiling syrup with a digital kitchen or candy thermometer and aim for 230° F. Or a few degrees lower if the recipe you're using calls for excessive peanut butter (more than a quarter cup).