How long should I expect my 18650 trustfire battery to last before I have to buy a new one? I vape the SB w/the 3.7 volt protected trustfire rechargeable battery. I have two, that I have been constantly rotating for the last 2 months, and it seems like they are going bad already, I thought they would last longer, I charge them regularly as suggested, but I vape pretty heavily. Is this the normal life-span?