it is stated as 2 ml or greater, but number is misleading.
DC carto must be always topped off.
Upper coil is placed well above middle of carto. Since juice will naturally flow toward carto's bottom, upper coil will dry out once carto is less than 2/3 full.
Dry coil will overheat, burning juice and flavoring residue and causing permanent damage to wicking material => permanent burnt taste.
On contrary cartos with bottom coil such as Echo can be used much longer
Got some Echo cartos today, from Mountain Vapor at http://www.mtnvapor.com
Out of all cartos i've tried (a lot! Boge, Boge LR, Joye Mega, Clearomizers, Giants, Vortex) these are the best. Taste, TH are good. Others say they do not leak, we will see (so far, all cartos i've tried were leaking at some point)
I'd say these are Joye Mega cartos done right. Joye were made of plastic, PITA to fill, and they leaked like crazy from both ends because of weird airways.