There is no simple answer and everyone has different requirements. When using a device above 40-50 watts (sub-ohm vaping) it is strongly recommended to start around the average of 3mg/ml. Sub-ohm vaping causes more e-liquid to be vaporized at a higher temperature, providing more nicotine per puff when compared with using the same juice with a non sub-ohm coil at a lower power. Sub-ohm vaping e-liquids higher than 6mg/ml may cause nicotine overdose if not used properly. When using a device around 8-15 watts with a coil resistance higher than 1 ohm the average set strengths for regular nicotine liquids are 3mg/ml, 6mg/ml, 9mg/ml, 12mg/ml, and 20mg/ml. 3mg/ml being for a light nicotine requirement, all the way up to 20mg/ml for the heavy users.