It will depend on the level of performance you are willing to accept. Most use 18650s not just for vapetimes, but for the high and longer lasting initial voltage. I personally do not want to vape below 3.4V. And note for the most part vapeable mAh with a ~2600mAh cell is closer to 1800mAh.
As a general rule of thumb figure about 300mAh per hour of the stated mAh for decent performance.
cskent and iamtopher34 both make very valid points. If you are a perfectionist after about 100 charge cycles you will notice a drop in both performance, lower loaded volts, and vapetimes.