Breaking it down

Here is some useful information to have handy. 

Not complete not applicable to everyone but a basic start.

Drops:
Drops are what comes out of an eLiquid bottle with a dropper tip. 
There are approximately 20-25 drops in 1ml.

Puffs (vape inhales) 
There are approximately 5 puffs in one drop, so about 10 puffs per 2 drops. 
Two drops is the amount most commonly added when dripping. Dripping is the art of dropping liquid directly onto an atmozer bypassing a cartomizer..

Typical users:
A typical user vapes around 3ml / day. You may vary, I know some vapers who go through 10ml a day!

So at 3ml per day for a typical vaper:
10ml bottle lasts about 3 days.
20ml bottle lasts about 1 week.
30ml bottle lasts about 10 days.

Nicotine Content:
24mg eLiquid means 24mg / ml.
1 analog cigarette = approx. 1mg nicotine absorbed.
1ml of 24mg nicotine eLiquid = 24mg nicotine... but read further......

The mg stated on juice is what is in the liquid. Only about 1/3 of that is absorbed, possibly a good bit lower (some tests have said as low as 10%; but the maximum is probably 40%). Maybe closer to 30%.
A box of 20 cigs gives aboout 20mg nicotine (20 x 1mg).
2ml of 36mg liquid will give about 2 x 36 x 0.33 = about the same: 20mg absorbed.
So 1 box of analog cigs is roughly equivalent to 2ml of 36mg juice or 3ml of 24mg juice.