Are you finding you only get 2-4 tanks off a single carto? I was seriously considering a tank from ibtanked but not if I'm going to be going through replacement cartomizers like there's no tomorrow. I go through 4 tanks a day at least.
Right now with my vivi nova I'm changing coils weekly. They can go much longer, I just like to have a fresh one. I have 100ft of wire and a bag of cotton balls, all of which costs under $10, that will last me a year.
I don't know how many tank fulls I've gone through but I'm still using the same carto since 10/20/12, so that's about 12 days. Don't have a very good reference, I just remember when I got the tanks and only changed one carto because I mixed the wrong juice in the tank.