The average vape coil lifespan should last around 1-2 weeks, but this is dependent on a number of factors.
MTL Coils tend to last longer than sub-ohm DTL coils,
in addition the coil lifespan is dependant on:
Type of tank you have
Quality/ brand of coils
Voltage or wattage you vape at
Rate at which you are vaping your e liquid
Type of vape juice you are vaping
Strength of battery power used
3 reasons why your coils are not lasting as long as they should:
You are smoking dry hits - your coil will be burning its wick instead of your vape juice
You are using particularly thick and sugary flavor concentrates, which will produce a higher build upon the wick and coils.
You are vaping excessively, the more you vape the more you wear down your coils, so if you are a consistent vaper,
look into different vape juices that are easier on the coils, or change your coils more regularly.