It depends. If you don't vape heavily a carto can last all day. If you do vape heavily, you might end up refilling it a couple of times.
Don't sweat it too much if you don't totally dump regular cigs the moment you get your PV. If you really want one, have one. After I got my PV, I still wanted a regular smoke at times for a few days. Right away in the morning and right before bed mainly. I didn't beat myself up over it and now I've been free of the analogues for 5 days now.
+1
Completely agree. You won't know how long a carto lasts until you start vaping, and see where you are with it. The nic level also makes a difference - you'll vape less with higher nic because it will satisfy your cravings, but too much nic and you'll start feeling like your heart is beating too fast or will get a slight headache. Listen to your body and be prepared to adjust your nic levels and how much you vape. Drink water!!!
I also agree on not beating yourself up if you have an analog. When I first started, I was smoking about 4 or 5 cigarettes a day, and tapered off to nothing over several weeks. I gave myself permission to smoke if I really felt I wanted to so that I didn't feel like I was depriving myself. Vaping took over - I didn't care about smoking anymore.
But if you run out of juice or you drop your battery in the toilet or your dog buries your charger in the back yard or whatever (you will quickly learn to buy backups of everything, and have backups for your backups!), and if you smoke then, it's not going to send you right back to 27 packs per day. Vaping doesn't work that way. If you take to vaping, cigarettes will actually stop being satisfying to you and having one will not make you want to start smoking again.