If you are not going to add some juice to the cartomizers while vaping, then you should keep a whole pack (5 pcs) in your pocket at all times; you never know when they quit and out of juice. Really, until you vape them for a while. Sometimes, you may vape for an hour stright and sometimes you may vape occasionally.
The first time I started vaping (KR808 Cartos) I used 5 cartos in 2 days. I bought the starter kit from a local cigar shop, and it came with only starter 2 cartos. They also had the 5-pc/pack cartos for $19.99+, so I bought one pack, thinking it should last for a while. Two days later, I was out of the 7 cartos (5+2.) I drove back to the place and I bought another pack; when I realized I should go back to analogs; they were much cheaper.
Then, I started buying cartos online for less than half the price, until I had hundreds of empty cartos in a box. I did not want to throw them. I learned later that I could have extended the life of a cartomizer with a small juice bottle in my pocket. Every couple of hours, drip a couple of drops and the cartomizer could last you at least two days before the taste starts to change.