Greetings.
I am currently a fan of the tea bag method: the draw is fabulous, filling and topping up are quick and easy (even more so if you use a syringe), and the cart stuffing process is ridiculously simple. I tend to fill carts with liquid in a batch of five or six, swap them out with full ones as they need topping up, and then top them up (with syringe) in a batch, so I can't really claim that a cart needs to be re-stuffed every x days. My best (conservative) estimate is that a single tea-bag stuffing would be good for maybe two days of continuous use. If that seems very short, bear in mind that it only takes a minute (two max) to prep a tea-bag stuffing plug and the only tool required is a part of sharp scissors.
I use these empty (Finum brand) tea bags here. I am a long-time fan of Upton Tea and everything they sell:
Upton Tea Imports: Teaware - Cup/Mug Filter Bags (AI42)
Here is a brief summary of my procedure for stuffing 510 carts with tea-bag material:
1. Trim and discard the side seams. The fastest way to this is to fold the bag in half one or more times, just to shorten the length that you need to cut thru with scissors (think of strings of folded paper dolls to get the visual).
2. Cut the bag in half parallel to the side seams that you just removed. When unfolded, you should have two long strips of material, enough to fill two carts. I usually tear off and discard about two inches off of each strip to get the right amount for a 510 cart.
3. Start at one end of the strip and begin folding the paper up into a plug. I use my fingers back and forth to do a sloppy accordian pleat, as I feel that the pleats result in an airier structure than rolling it up like a carpet. Make your pleats roughly as wide as the intended mouthpiece.
4. Tidy up the plug into a shape and size to fit your mouthpiece, then trim a small bit off of one end to make it flush for the bottom end of the mouthpiece.
5. Slide the plug into the mouthpiece all the way, then pull it out about 1 mm, and trim the excess off across the open end of the mouthpiece. Tuck the plug back in, and you're done.
6. Fill that baby up with liquid--the fastest and best way IMHO is to fill from the bottom up using a syringe.
Please give a holler if I've been unclear or if you have any questions.