Then you have another problem, like a parasitic drain when the vehicle is parked, or a diode out of your alternator. My 2004 K1500 GMC Pickup is driven infrequently (once a week, on average) and still has the original battery. My 2006 Toyota is driven daily, and still has the original battery.
I tend to very much agree with MikeMI on this. Now I don't have always on headlights but on many trucks (I guess cars also) I have noticed that when cranking to start the headlights actually are cut out (off) during crank till the key is released. My truck doesn't do that and I just got the wife (who now thinks I am nuts) to stand in front of the truck while I had the headlights on and started the truck. The lights remained on.
I generally get 5 to 6 years on a 5 to 6 year battery also. When they finally do fail it seems always the coldest day of winter!
Ron