I've got a mega tank but hardly ever used it .I started off with the aero
tanks and have about 5. I used to buy double coils but realised pretty quickly it was expensive when I only averaged 2-3 days/ coil. Discolouration of the liquid in the pyrex tank was indicator enough for me to change the coil. Some liquids cause more gunk than others along with degradation of flavour. My wife only needs to change coils once a week as she doesn't/ can't vape at work during the day.
I saved all my shop bought coils over the last year and more. I switched a while back to the
protank 2 singles, I think it is. First I started re wicking them which then lead to recoiling. That's the best thing I ever did as they cost me next to zilch now, just wire (30 or 29 AWG) and some KGD or Rayon.
I wont be buying shop coils ever again.