about 5-6 ml of 12, 14, 16 or 18 mg a day, depending on which diy juice I feel like. No insomnia or other side effects. I used to be very prone to catching every virus kids brought home from school, and this fall I haven't had anything (knock on wood). Dunno if this is vaping or dumb luck, but not complaining
I've noticed the same thing. I do notice that after a 20 minute vape break at work, that my sinus passages will start to run. Generally will have to blow my nose once or twice during break, but once on my game, no nose run. When I vape at home, no major issue. Thinking that since I work in a smoking environment, my vape must be cleaning out all the second had smoke inhaled during the time spent out on the floor.
I started with the pre-filled cartridges (mostly 24 mg) and would easily burn through three to five per day in addition to smoking a pack or more or regular cigarettes. I learned my lesson, switched to e-juice, and as of the last few days, I have been going through about 3-5 ml (24 mg) per day. During my first week with e-juice, my peak was probably at around 8 ml.
I have yet to notice any side effects from vaping, except that I need to keep myself well hydrated at all times. However, when I quit smoking, I coughed my lungs out every morning for the first few days, couldn't sleep, and had massive headaches. Now things are starting to improve.
The great thing is that I am almost at the point where I would never dream of smoking another regular cigarette again. I passed a group of smokers while walking today, and just the smell made me almost gag.
Thinking back on the last 7 months that I've been a vaper, the only discomforts I had at the beginning of vape, were smoke related issues. I was at the point that it hurt to smoke most of the time. So just recovering from that, and detoxifying from 27 years of smoking chemicals and pollutants which had their own pre-existing factors.
I vape about 1-2 ml of 16mg liquid a day. My setup is a 900maH ego passthrough with a smok tech ARO Pyrex tank on top. Absolutely perfect, and I don't experience any side effects.

I do start to notice when I'm chain vaping a bit much.
The amount of juice consumed a day is highly dependent on what ones vape gear is and the watts being vape at.
I have two pvd's, and several rba's.
I have a Provari, which I run at the highest watt/volt setting possible for my build, which is about 14-15 watts, depending on the build. I just keep adjusting the volts up till I get an E2 error, then dial it back .1 volt.
On the nemesis mechanical I am currently using a .5 ohm build, so that would put the watts at 30 something. When one vapes at high watts, juice is atomized at a higher rate. That is one of the reasons I went to DIY juice, to offset the cost of vaping 10-15 ml a day. so that comes out to about $1.20 for juice a day.
I mix all my juices at 12 mg nic, 80% VG, and 8-13% flavor. When I started, I used 24 mg nic, as my gear and builds improved I had to drop my nic level down other wise I'd get way over nic'd.