I never ever run them empty because the stuffing will singe and make the carto bitter. Whenever I think the vapor or flavor is light or tastes off, or if the carto is getting abnormally warm, I add liquid. I also use a plastic or delrin drip tip (mine are from LitecigUSA.net) in place of the end cap so it is easy for me to pull that out and look at the stuffing. I only use juices that are thin (PG w/ 20% or less VG and 2 Vapor4Life WOW juices) and I avoid juices that drop a lot of flavoring sediment (clogs cartos, ruins drag) so my stuffing is left pretty white as the juice level gets lower. So I can see if a carto could use more juice. I make sure to check before putting a fresh battery on, before leaving the house and before bed plus whenever I think I have been vaping long enough that it may need juice.
I still occasionally singe a carto a bit but a very sweet juice may be able to overpower a bit of bitter singe and some tobacco juices actually taste OK with a bit of singe. But if you singe one slightly multiple times you will end up not tasting the juice at all. I can get 7-10 days use out of one carto.
Also I never ever put untested juices in cartos. If you have a manual battery you can put a standard resistance DSE901 atomizer and drip tip on your battery to test new juices. If you bought auto batteries then drip tips are not recommended - I guess you could use cartridges or just be extremely careful with dripping. And if you bought autos or even if you bought manuals, it is a good idea to have a 3rd battery. The 3rd battery can be a used as a spare, as an overnight battery (what I do so my ecig is ready to vape in the morning), and if you have autos your spare could be a manuals used with atty and drip tip when testing juices.
You can also use cartomizers from Bloogplanet, Halocigs G6, Vapor4Life
Your model is KR8708D-1 threaded and the Bloog style (they were the first with that style battery and Vapor4Life is more like the original manufacturer ones)