Well,
I haven't heard that if you take Birth Control for a long or certain about of time, it stops working. A lot of Women stick to one B/C for years.
Spotting is normal, but if you get strong breakthrough bleeding and other symptoms, then you might want to see your gyno.
I'm not sure if the whole Birth Control not working after time is correct, but I'm sure for some bodies, sure maybe it can quit working. But like I said, women take it for years and years and it doesn't quit working.