Research suggests that the average person needs about 15 exposures to a new word to effectively remember it. That's for a native speaker, but language learners typically require more than twice that many exposures. Those with an extensive vocabulary in the target language will require less. Different studies have suggested slightly different numbers, but they all seem to be roughly the same. Of course those are averages, as well - some need more, some need less.
However, it's not just repetition alone that matters - variety in exposure is just as important as the number. Variety in expose types is the most beneficial.
Different types of exposures would be reading the word, hearing the word, writing the word, using it in original sentences (both spoken and written), etc. Even within the same basic type of exposure (i.e. reading), variety is better. Reading the word in a sentence is slightly different than reading off a flash card or study list, etc.
That being said, and research aside - I almost exclusively learn vocabulary by reading, at least for the initial introduction to a word.
