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.