4 years and enough cv's and covering letters to fill a small caravan. Eventually changed tact just as things perked up and ended up with five calls for interview. 'Keep at it, it will happen' is, in my humble opinion, not necessarily great advice. If what you’re doing isn't working for you, think about what else you could do, or how can you change what you are doing.
I made the mistake of forgetting that the people I was applying to are humans, and recruit similar people.
It certainly helps if you have a friend pushing your cv from the inside, however I had no such help so don't worry too much if you don’t.