pre made coils like you are using will vary
they are mass manufactured so you will get some that last longer then others and some that will die quickly
as others mentioned letting the cotton get dry while firing it will shorten its life span. this can happen form no juice in the tank, firing at too high a wattage for too long so all the juice vaporizes etc ect
The type of juice will change it as well
some flavors (bakery,desert, sugary) cause a build up on the tank and in the cotton which can happen in days even at 2ml a day as you do. while flavors like simple fruits can go and go and ago and go without causing any to very little buildup in the coil and cotton.
On average from everything I have read and discussed with people IN GENERAL 2-3 weeks is a norm on juices that dont cause significant buildup and you dont do anything to damage the coil or cotton. Now that is not to say 4-6 weeks can not happen. I have had coils last that long before.
Juice that cause a build up will make a coil last much less sometimes days But since you only vape about 2mls a day you are going to get much longer life then many of us who are vaping 10-30mls a day. It is about how much juice not time.