First, the whole thing, trying to equate liquid to cigs etc.....throw it out the window and don't think about it again.
I don't know what you vape with so my help won't be all that good but this is how I like to use my cartos.
Tried lots of them. Prefer the soft top 510 cartos. I use a drip tip so the soft top hits the garbage immediately. Fresh carto, about one ml. I top off regularly. When, well, it depends. If I am sitting at my desk often, if I am running about I'll vape them down some. When I top off, I drip the liquid down the sides, not in the middle. get the filler good and soggy looking. If the top of the filler is starting to look dry and fluffy, even if you have vapor production, that usually is a good sign to top off. These 510 mega soft top cartomizers, I am partial to, so partial they are the only ones I use now. I can't say enough and that is because finally, I found my happy spot that keeps vaping just as easy and more satisfying than an analog ever will be. Everyone differs but when you find the right pv and atty/carto/juice combo it is wonderful, it is at that moment you quit thinking about cigs.
At first I tried to work my cartos forever, even went so far as to do the whole cleaning thing. Good if you like to vape a large variety of juice during the week. I stick to about two flavors a week and now just treat myself to new cartos once a week.
As far as comparing life to cigs. It is a good advertiser to get you in the door. Once you start to hit that middle point between vaping and smoking and key into all the tips and stuff that work for you as a vaper, then honestly, the way to get completely over the hump is to stop comparing. Its similar but so different than smoking. Never did I find something that tasted or inhaled exactly how my smokes did. Once I made it past the hard point then I just started to look for a device, a delivery method (like cartos) and juice that kept me happy.
How much juice? A lot of this depends on your vaping device and voltage. I use a mod. At 3.7 I don't burn up near the juice as I do at 6 volts. I want to say I go through about 3mls a day. I never did measure exactly and I do know some gets wasted when I over fill and need to blow out excess liquid. I also vape quite a bit, maybe more than I smoked but have reduced my nicotine levels down considerably so now it is more just hand to mouth habit than nicotine addiction.
Best thing I can recommend is to go read the reviews on devices and such. Do searches because alot of good ones end up pages and pages back. Then, go into the vendor forums and read too. Once you start posting in other sections, don't be afraid to go into the vendor forums and ask questions. All of them have loyal followers and will be happy to give you the tips and tricks for any particular item.
Saving money, another great advertiser to get you in the door. I spent a ton up front and did a lot of trial and error. It's all good though, I am done with smoking. Six months into the vaping and now I am at the turnaround to start recouping my dollars in regards to what I used to spend on smoking. (but there are many tempting new pv's juices etc that might prevent this! LOL)
Not sure how helpful this was. I remember how confusing the whole process was when I first began but a veteran steered me to a battery mod, the members who posted in the vendor section helped me with the whole batty/atty/carto process. That was over three months ago. That was when I smoked my last cig and never looked back.