So tell me, how are you getting a week out of cotton and not losing taste. I rebuild and use cotton but I can taste the change in a couple days and re-wick.
I'm sure the juice plays a part in it, but I haven't really noticed a big difference.... I think honestly a lot of it comes down to build... I remember when I first started trying to use cotton (in a kayfun) I hated it. Thinking back now I'm pretty sure it was just that I sucked at building! Too much cotton and it doesn't wick right. That means dryer hits and more wear and tear on the cotton. On the other hand, too little cotton and wicking is fine, but the coil just isn't soaked in juice the way it's meant to be. Plus if the coil isn't good, and you have any hot spots or un-eveness in the firing that plays a part too. I haven't used silica for a long time, but I remember after just a tank or two of juice I would pull the coils and it would be a nightmare... The coil would be nasty and gunky and the silica itself was just blackened and horrible tasting. Again I attribute this to the fact that rather it's my build skills or what, I just can't seem to get silica to wick right. Certainly not the way I can cotton.
When I build with cotton, I'll take the time to unroll the ball, examine the grain.. look for a good "spot" to pull the wick from... cut it out and then roll it until it's what I deem "perfect". I like my tails to have a slightly larger bulk while the part that goes through the coil is slightly thinner. Not too thin though. The idea is to keep the coil well supplied with juice, and have plenty in the wick as a reserve, yet not "packed" in there... Remember it will expand slightly when wet, but in the same respect you've got to make sure the coil has complete and full contact with the wick.
I can run tank after tank after tank this way with no problem at all... When I finally do change the wick the coil is still pretty clean and while there is some darkening around the wick it's NOTHING like it was with silica. It's really just that, a darkening of it. Next time I do a wick swap I'll have to try to take some pics and see if I can post them up. My coil in my kfl has seen heavy use lately (i've been using it every day pretty hard) and is probably a week old at the moment... So I'll see if I can re-wick it soon.
Like I said, I think it's just a matter of getting the wick right.. So it's wicking at a speed that can keep up with the vape and not have hot spots or un-juiced portions of the coil... Any of those factors will burn the wick and deteriorate it's performance which leads to a kinda downward spiral...