Coking starts immediately if the power on coil surface (heat flux) exceeds 0.17 W/mm2. In case of NETs everything depends on this. Unlike synthetics organics doesn't like heat. For example on Taifun GTR with KA1 28ga 6x2.5mm on 9.5W I need to change cotton after 12-14ml. On 10.5W (0.18W/mm2) cotton lives about 2ml. The other setup depends on extremely low energy. I'm using 50PG/50VG/5AD ratio with 5% of traditional NET flavours like Black Note. Water lowers boiling temperature. New generation of organic flavours like La Tabaccheria 4Pod or TNT Distillati Puri seriously increases coil life and permits to use 50/50/0 ratio with 10%. But still requires low energy setup.