1. After my initial dumb move of not waiting after a first time fill and burning one out... I have been waiting 2+ hours on initial fill before using one... is this overkill? how long should I be waiting? Yes, that's overkill. I go by the 30-minute standard, which is probably overkill also—but better safe than throwing up into next week.
2. A couple of days ago I wanted to vape a different flavor, so I took out the one I was using and put in a new one. How long can that one sit, will it dry out? When used again will I have to wait a few hours for the liquid to soak in again? I just want to avoid that horrible burnt flavor that came from using to soon... if I never taste that again it won't be long enough. I'm not sure about this one; it will dry out, of course, but will be reconstitutable? Don't know. Cartos are relatively cheap.
3. On average how long do these 510 boges last. I only have 3 unused one (for me and the husband) and would like to buy more for a rainy day. YMMV. I get a week out of them.
4. How does one test new flavors with these. I assume the liquid that soaks into the filler sticks around for a while, so if I dumped a new flavor in I would be tasting a mixture of the two, and not really know what the new one tastes like... Does this mean I have to keep some of my old school 510's around to drip a new flavor on before I use that flavor on a cartomizer? If by "old-school 510s" you mean atomizers, the answer is yes (although I'd recommend you try IkenVape's Cannon Attys); dripping is good for a lot of reasons, and sampling flavors is one of them—although you'll find things won't taste exactly the same when transferred to a carto.
5. Does everyone keep a backup cartomizer filled in the event that the one is use stops working? I can't imagine going two hours without vaping waiting for a new one to soak up the liquid. This brings us full circle back to question 2... If you do have a backup will it dry up? I don't, for the same uncertainty I mentioned before—and for the 30-minute soaking time I use; I can stand to wait that long.