How frequently you replace coils really depends on how much you vape. There is no definite guideline on how long a coil will last and there are several factors affecting the life span of coil such as the type of liquid and the wattage or temperature you vape at.
You will know when your coil needs changing as the flavour of your eliquid will reduce and you will start to get a burnt flavour coming through.
Heavy Vaper:
Someone who uses their kit continuously throughout the day may need to replace coils between four to six days
Medium Vaper:
Someone who vapes regularly may find coils can last up to fourteen days
Light Vaper:
Someone who uses it socially or sporadically throughout the day can see coils last up to three weeks.