I didn't read all the replies. I bet all your questions were already answered. I am new here also only been at it for about 2.5 weeks.
I have read tank carts will last about 30ml-60ml of juice it varies. With my first tank cart it seems to be losing power (still works) and I am almost through 30ml of juice.
I was just under a pack a day smoker before coming to vaping. I have been vaping very liberally (in my opinion) I have gone through 30ml of juice in about 3 weeks. I imagine others will go through juice faster or not as often.
I really prefer the tanks to the carts. I like being able to see how much juice I have left and not fear to burn out my cart.
Hope this helps a bit.