This is how I fill and keep damp standard 1ml cartomizers like Boge 510 and most KR808D-1 cartos
I only use thin clear juices to get the best life out of each carto. I use PG w/ 20% or less VG and I acoid all dark/opaque/cloudy liquids that drop flavoring sediment because that stuff will gunk up coils and turn the stuffing to pudding. f I keep a carto damp so it doesn't singe and get bitter, I can run about 20ml of juice through a carto before I know it's degraded enough that I want a new one. For me that is about 7-10 days or more. nd since I alternate vaping 2 models that's 2-3 weeks for me.
What I do, based on how long my own cartos last, is I make the carto give me 7 days of use (which in my case is 2 weeks since I vape 2 each 1/2 the time). Then on the weekend I decide if the carto is working well enough to give me another week. If the drag is getting hard and it's not giving vapor easily, then on Sunday I fill up a new one, settle it on end and on each side, and then switch and I throw out the old one.
---
The carto has a 1ml capacity. You need to aim towards putting 1ml in. I hold a tissue on the threaded end hole (or put a carto cover's spike in the hole), tilt the carto and drip in 18 drops (waiting longer for the first few to soak in), Then I wait 15 seconds. Then I see how many more it will soak in easily (usually 3-6, occasionally more).
Then I clean it up - clean threaded end, tilt open end down into tissue and puff into threaded end to clear the air channel, then while still pointed down I just touch a soft bit of tissue to the top of the stuffing to sop up excess (to help prevent leaks).
Then I let the carto settle. I use juices with 20% or less VG so 15-20 minutes of laying it on different sides and on end is plenty of time for every bit of stuffing to get damp.
While vaping - any time the vapor seems light, dry, or tastes the tiniest bit off or loses flavor, or if the caro is getting abnormally warm, I add liquid. I also occasionally look at the stuffing (especially before putting a fresh battery on or before going out) and if the top of the stuffing looks white, I add some juice. No settle needed when just keeping the carto damp. And if you do not keep it damp or if you use juice that is too thick (too much VG) you will singe the carto and that adds a bitter taste. I use a simple plastic or delrin drip tip in place of the end cap so it is easy for me to remove that to look at the stuffing or add juice.