In order to get your message into long term memory, it must be repeated seven times. However, marketing communications only 'get through' one time in three. Therefore, you must try 21 times (i.e. 3 x 7), in order to get your message across.
Consider for a moment those companies which mail you on a regular basis. If they continue to do it – it must work for them, mustn't it? The chances are that you have bought from them in the past. This means that you are likely to buy from them once again, at some point in the future. The point is that they don't send a few mailers and then give up. They are investing for the long term - and so should you.
All the best with your marketing!