When the syrup reaches full boil with big bubbles, just let it boil for about 5 minutes. (eugeniekitchen.com)
Ok, I made the icing last weekend... mine ended up runny - though I followed everything to the letter... wondering whether I didn't let it boil long enough.. (elanaspantry.com)
Bring to a boil and let it boil until reduced to a quarter of the original volume. (userealbutter.com)