If you use and atomizer and cartridge (aka cart, the way they come) they might come with some prefilled carts but prefilled carts don't last long and can get dry on top before the
juice is gone (and you don't want to vape dry because you'll get the burnt poly stuffing taste and possibly strain your atomizer. You want the atty to stay damp. So when the vapor lets lighter or drier or you get the slightest bit of burnt taste, check the cart, top if off with some
juice. The top of the cart stuff should look damp but not a puddle.
When the white stuffing gets old and cranky - tastes bad or not wicking well, you can try washing and drying it and putting it back in but I think blue foam is easier and works well. Buy the really cheap precut 510 blue cart stuffing at strictlyejuice.com for the cheapest easiest way to try blue foam. With blue foam it is easy to remove and put back, you can rinse it under running water and pat dry, You can do a full fill by removing the stuffing, filling the juice cup 1/2 way, putting the blue foam back in (tapered end first) and push it into the juice, reposition at the top edge of the juice cup (an unbent small paper clip works well) and top of with 1-3 drops. A full fill should last longer then you can top off when needed.
Carts need to be topped off with juice the most often.
Your other options besides atty + carts
1. cartomizers aka cartos - these are combined disposable juice cart and atomizer. They hold more liquid and need to be topped off less often. I use soft-cap stuffing-type standard resistance cartos like Boge soft-cap. You need to fill a carto well to start so all of the stuffing is wet (20-30 drops depesding on the size of drops) and let it sit a bit - clean it up if it dripped, I blow into the threaded end to clear the air channel, maybe wipe up any liquid above the stuffing that won't soak in. let it sit for a bit (I lay it down on different sides) so all of the stuffing will absorb liquid. Then recap and start
vaping. may be a bit flooded to start - keep trying. but if you get a burnt taste stop - the stuffing is not all wet. Then when the vapor gets light or drier in a way that cannot be fixed by tilting the ecig carto down for 5 seconds, you remove it from the battery add more juice (5-8 drops, and however much more it wants), clean, blow, cap and vape. You need to add juice to a carto much less often than to a cart. This type of carto can last 4-7 days, sometime more. You can sometimes extend the life by straightening the center air channel if it gets crooked and is blocking air flow.
There are also no-stuffing reservoir cartos that need to be filled with a syringe.
2. Ego tank atomizer and tanks - a special atomizer that uses tanks of liquid
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For whoever ripped my battery estimate. I vape a lot more than 1 drag per hour. I take 3-6 drags divided between 2 PVs every 10 min or so. I started using fresh batteries at 12 noon - a 180mah 510 battery and a 280mah standard KR8. Used them at my desk all day at work, the in car, now using them at home - it is now 9PM and neither has died yet. I expect one will soon but that's 9 hours - but say 8 hours because I spent 1/2 hour eating lunch and 1/2 hour sitting at the dinner table. I am using a WOW cart on the KR8 and a Boge soft-cap standard resistance on the 510. Edit - as I expected, the 180mah 510 battery just ran out of juice.