Well, it's a tough question to answer because of the absorption rate of nicotine from vaping. So far there is no set percentage for that.
So, what I'll say is in theory based on what research I've read so far and the status quo.
Here is how much liquid people vape in a day:
We measure intake by ml rather than by per inhale.
It's easier to do this because the nicotine we're taking in is also measured by ml. If you look at a bottle of liquid it will be in a mg per ml rating, something like 24mg or 12mg. That is per ml. So if you vape 1ml of 24mg in one day you will absorb at most 24mg.
Ok, so that's not so tough but now we have absorption rate to consider:
One cigarette has around 10mg of nicotine in it to start with but you actually absorb around 1mg of that which is where the nicotine content rating comes from on the cigarette packaging. So there's your first guidestone:
one cigatte = 1mg absorbed.
As I said before we can tell how much nicotine we get by how many ml of liquid we vape but that does not take into account absorption rate. If we assume that the absorption rate is the same as a real cigarette then that is 10%.
Therefore, your 24mg would be 2.4mg per ml actually absorbed which would be roughly equal to two and a half cigarettes.
You can scale that for any liquid, 12mg would be around one cigarette per 1ml etc etc
I don't find it works that way. I use around 8mg and vape around 2ml a day. That means I should be getting about 1.6mg of nicotine a day which isn't even two cigarettes. I used to smoke 20+ rollies a day. That's a dramatic step down, but I can't vape any higher without getting symptoms of nicotine overdose, which to me is like having way too much caffeine in one day.
So it varies depending on you and how much you asorb beause I see some people use 52mg (I do not reccommend anyone vapes that much nicotine, ever) and some like me using barely anything and it's ok for them. You can base it off the mg per ml rating then apply a 10% absorption rate but it might not be that way for you.
Best way to know if you're getting enough nicotine is to listen to your body that's all there really is for solid evidence by the looks of things at this time.
Edit: long post is long
TL;DR vape enough until you feel you don't need to get more nicotine but don't vape so much that you get overdose symptoms and you'll be alright. Just listen to your body as there is no set in stone amount of nicotine you will need.