lol, well how long do you want it to last?? As long as one's not getting over nic'd, it's up to you. Frankly on work breaks, I vape as much as I desire for 20 minutes, while reading and having a cup of jo, and jamming some music. It's done when the break is done. Do have a dripping tank system, that more simulates smoking, push on drip tip twice, and have a good vape session, once gone, done with that vape, unless I want to push some more juice in.
Just listen to your body, and vape enough so that you don't have to light up. Every thing else will work its way out. I don't smoke, I Vape.
Vape long and Prosper.!!!