In the last couple of months, I've gotten a couple of Silver Bullets (love them). Anyway, like most others, I started vaping with a cigarette style ecig, graduated to eGo's, etc. Seems the lifespan of these smaller ones would run about 6 months before having to be replaced. How long might I expect a Silver Bullet to continue to function? I know the batteries may need replacing at some point, but how about the device itself?