I tend to rotate 5 cartos for approx 5 weeks (not dedicated vapor - still smoke, but under a pack per day now from almost 3/day, and I'm greatful for that !)...
My deciding factor to trash them is when they start to give "me" a funky taste (almost what you'd imagine a wet dirty sock to taste like, lol). I don't wash carto's (aside from simple alc wipedown), as I couldn't be bothered at under $10 a pack of 5 for 4-5 weeks of use. It just isn't cost effect in comparison to what I was experiencing in spending on cigarette's over that period of time.
btw, if it makes any difference, I use Bogetech 510 carto's.
VapeOn,
Greg