That depends on the type of website. My hairstyles blog rose to 500 unique visits per day with only 8 accounts in about 20 days but the same strategy is considerably less effective in the case of e-commerce stores.
For a content website, working on Pinterest creates rapid results and, at least with my content website, the Pinterest traffic does not decline; it is ever increasing.
Till now, for e-commerce, Pinterest has produced moderate result, say 1/5th of what Pin would produce for a content website. May be this is because I am new into e-commerce.