Depends.
In my VV Woodville, I can get through about ~5 ml on one pair of 18350s with a 1.5 ohm atty around 3.9v. I vape pretty close to 6ml per day, so if that's the only PV I use all day, I end up swapping batts once. I never leave the house without fresh batts installed, so I never carry batts with me. I use 700 mah 18350 AW IMR batts in that one.
In my 14500 REO mini with a 1.5 ohm atty, I need 4 batts and an extra juice bottle to do 6ml in a day. I could probably make it on 3 batts, but that last ml or so would be pretty poor performance. I use the 750 mah black AW Lion 14500s in that one.
In my REO Grand, with a 1.1 ohm coil, a single 18650 will drive it with acceptable performance easily through 6ml... But, since that's the mod with my cool new rebuildable atty on it, I've been swapping batts somewhere around 4ml to keep it at peak performance. I can easily get through the day on one batt, but I choose to use two. I use 2000 mah AW IMR 18650s in that one.
I will on occasion spend a day dripping 6ml in a 2.5 ohm atty on my 5.0v triple-v. It takes a pair of 18350s... and it is a battery hog... and I use some batteries in that that aren't top notch. 6 ml = 3 pairs of 18350s and sometimes a 4th set need to go in to get through the last half ml or so. I use some falsely rated 800 mah MNKE IMR 18350s for that mod. 600 would probably be a proper rating for those.