Without trying to offend anyone, how many ml a day is totally dependent on your mods:
Right now I'm vaping a single coil Kayfun 4, 1.2 ohms @ 13 watts .... this thing drinks juice, I need to refill the 3.5ml tank every few hours.
Before that was a genesis style mod, single coil with foil wick, maybe 3 ml every 4 hours.
Protanks, viva novas, cartomizer style tanks, etc ... a small tank would last me almost all day long.
Add to this dual coil drippers, they get real thirsty too.
Its not how many ml a day. Its how much nic.