The only thing I can think of is that you have a lose socket and as you drive, the bulb is flickering on and off many times causing it to heat and cool.. This will shorten a bulbs life span. But every few days?? I don't think that's it.

Does it actually work when you first put a new bulb in or are you just noticing it burnt?

The only other thing would be too much voltage But as Larry said, all bulbs would be popping.

Hmmm. you are buying #1157 bulbs? Because the only other thing I can think of is that you have a bulb for a 6 volt system or something to that affect.