Unless the potion blows up, it is ready and usable as soon as the villager puts the last herb into the cauldron (white steam will be coming from the cauldron). Assuming that you're using the correct combination and the potion didn't blow up, perhaps you're waiting too long to try to use it? The potions evaporate within a short time (tap the cauldron to see how long the potion will remain), after which you will need to make a new one.