If it's a KR808D-1 1ml-ish carto... A carto lasts me about 5-10 days by keeping it damp. I never run them dry because if you run a carto dry you can singe the stuffing and ruin the flavor. I use a plastic or delrin drip tip as a mouthpiece on my main cartos so it's easy for me to take the carto off the battery, pull the drip tip, and add a bit of juice carefully into the stuffing. Then I clean up and continue vaping.
How often it needs juice added depends on how much I am vaping. I usually alternate vaping 2 cartos and add juice to each several times per day. I am a moderate vaper. When I feel a carto needs juice, it usually ends up needing 5-10 drops that it will soak in fairly easily. I feel it needs juice when the vapor tastes a bit light or a bit dry or a bit off or if the carto is vaping too warm.
With blanks...
When I fill a blank, I hold a tissue on the threaded end hole and make it take at least 18 drops per ml of capacity (I tilt it and drip juice down the inner wall into the stuffing). Then I wait 15 seconds or so, then add however many drops will soak in. Then I clean it up (clean dry threaded end. the central air channel if it has one, I will clear by blowing into the threaded end. If the stuffing looks sopping wet I touch a tissue to it because I don't want juice in my mouth). On a fresh fill of a blank I then let it lay on different sides for 15-20 minutes to make sure every bit of stuffing is damp before I try to vape it.
I find I get the best life out of a carto w/ thin clear juices, 20% or less VG and light clear colored juices that leave the stuffing looking pretty white and don't leave a bunch of flavoring sediment behind.
Note - if you have auto batteries (no manual pushbutton) then you need to be especially careful to avoid cartomizer leaks. If a carto leaks it will leak right onto the hole in the middle of the battery's threaded end. Behind that hole on auto batteries is the necessary drag-triggered switch autos have. If the switch gets sticky or shorted with liquid, it can ruin the battery. So avoid overfilling and clean up well. Also if you hear gurgling sounds or feel a bit of juice reaching your mouth, check for leaks at the battery connection too.
Crackling is an OK sound - nice new cartos can make a happy crackling sound when vaporizing juice.
Grats on your purchase - a good choice. You can also use cartomizers from Vapor4Life, SmokelessImage, Bloogplanet and other KR808D-1 cartos. KR808D-1 is the strongest slim ecig at a full 3.7v. If you find the nicotine level you chose has too much TH, try the next level lower. You won't have a nic fit - just take some extra drags. 3 drags of 16mg is the same as 2 drags of 24mg etc.
