Well NET is definitely going to last less time. I normally do not see any brown marks at all on my TC coil's wicks but the RTAs I run in wattage mode can get a few darker spots. I am not overly finicky and inherently lazy so I try and re-wick as little as possible. In fact I am probably overdue to re-wick my goon clone that gets pretty heavy rotation. There are guys in the Rayon thread that say they go 200 - 300 ml's between re-wicks.That's impressive. Any dark brown/black spots on your old wicks indicative of burned juice or rayon overheating? I personally have never seen a clean wick after a weeks use. I go by taste, when the vape starts tasting harsh I know it's time to replace the wick or stock coil. Typically 3-4 days with NETs or flavored juices. About 10 mils max haha.