I keep about four cartos in my rotation (four different flavors) which I swap throughout the day. I fill them all in the morning when I wake up.
I usually can refill them for about a week before they don't taste so great. At that point they go into the recycle bucket and I fill a fresh one. I haven't really boiled/reused any yet, but I figure that if I ever have a vaping emergency, I have a backup stock to pull from. Four cartos in a week is well within my spending range, so even if I was throwing them away I'd be fine with it.
Acorn, it gets better as you go, in regards to that brnt taste you mentioned. At some point you start recognizing that "finished" flavor before it tastes burnt, and you know when to refill in advance. It just takes some getting used to.