The answer relies on both frequency in short and long-term perspectives. Within a given class, a good number of repetition is between 5-10 times, this can be spread out during a class, but the total number will be in this area. Over the course of a week or a month the exposure of the word may reach between 50-100 times to really reinforce the learning of the word. It is also important to learn words in context, rather than in isolation or just by itself.
