I read a lot about how long coils last on this forum. I usually read 1-2 weeks for most people, with some people getting as much as a month out of one with vodka soaking and such.I never got close to that though.
To me, the Protank 3 is the Windows Vista of Kanger tanks. It should have never been created, because it's basically a Protank 2 with dual coils, but without the air control valve of the Aerotank.
I use Protank 2 single coils with an aerotank base, and vape about 2.5ml a day. As a primary tank, I never got a coil to last me more than 3 days, even with dry burning. It wasn't a big deal, as I would need 2-3 coils a week, at a cost of about $1 a coil, but it was annoying that everyone else I knew could make their's last a week with the same tank and equipment.
I got so irritated that I started to rebuild my own. I still rebuild them about every 3rd day. It takes 5 minutes per coil. I guess it's just my vaping style that kills coils quickly. Now that I rebuild them though, I don't have to worry about wasting a coil that still works, but isn't that great. I also benefit from the better wicking material.
When I still used stock heads I used 20/80 juice and 50/50, but the coils still last me about 3 days. I am a believer that dark juice kills coils faster though. If you went with a light colored fruit flavor it would last longer than a dark colored coffee flavor.
In your case though, I think you're either vaping at too high a voltage, or you're chain vaping too quickly. With stock heads, I would get burn by the third consecutive hit. With cotton and rayon you can chain vape away all you want, but you would have to learn how to rebuild them.