Just don't go by the "1 ml/1 carto = a pack" analogy that some suppliers keep trying to say. THEY LIED!
Well OK, it may be true, but a common trend seems to be that most people vape more than they smoked, so maybe 3-5 mls/day would be a good supply guesstimate to start?
A lot of people I've talked to (including myself - yes, I talk to myself) say that they plan to vape as much as they smoked, but keep in mind that this is a much slower/less efficient nic delivery system, and you'll probably have to do it more frequently to keep a steady supply coming (which is OK to do - just listen to your body when it sends you signals on nic intake). I chain-vape whenever I can, but it's not all bad because I've been able to lower my nic to 6mg, and can go for crazy long hours without it, and not go through any of the withdrawal symptoms at all that I would've gotten when I still smoked.