This is a "how long is a piece of string" question.
you won't start taking on additional nicotine. your body regulates itself via a process called "self titration" and when you've had enough it tells you to put the vape down.
the method that nicotine gets into your system from smoking and from vaping is different. A new user using lower powered device like an eGo usually finds it takes quite a few minutes for the nicotine in the vapour to get into your system, nicotine from smoking gets to your brain in just a few seconds. Experienced users with more powerful kit can replicate this almost, but you don't normally get the quick 'hit' that you do with smoking. You get used to this, so don't worry about it, just be aware that it's a little bit different.
As far as how much e-liquid gets used. I've found that for an average new user that was about a 20-a-day smoker then they'll get through about 1 EVOD full (or about 2ml) or e-liquid a day. Heavier smokers might use more, and lighter smoker might use less. Some people find they like vaping so much that they drop the nicotine so they can vape it constantly, if this turns out to be you then you'll use more liquid obviously. Some people use the vape like they used the cigarette and have a vape and then put it down for an hour or more between vapes.
Lots of more experienced users wind up using more like 2 EVODs full (or about 5ml of e-liquid a day) especially if you move onto to Genesis atomisers, cotton wicked micro coils.
Juice is pretty cheap though (especially compared to smokes) and you can get really low nicotine content juice if that's what you're after so you can vape more often and still keep your nicotine content low.