So glad you're liking your Volt - I'm planning to order mine this weekend, now that all the new stuff is in.
As far as cartos:
1) you can keep refilling until the flavor doesn't taste right anymore. I've been using horizontal-coil cartos from somewhere else, and they've been going strong for around 2 weeks. I'm only cleaning them now, because well, it's been 2 weeks (bacteria freak). I only clean and reuse because I'm really poor and need to get the most use out of my stuff. Like everyone else says, performance is never quite the same afterwards. Some I've cleaned 3-4 times before they didn't do it for me anymore, others only once. They vary - in production and in what the liquid has done to them.
When you clean, try to find the gentlest method you can (boiling, soaking, ultrasonic, etc.). I did the oral syringe thing once, and screwed up a bunch of them (worked great for a lot of people, but not me). I tried shaking them vigorously too, and they were clean, but never all that great. I think I' might've shaken/jarred something out of place inside. Lots of ways to clean them. Just search here or YouTube.
2) I've been using the condom method (from the mouth end), just because I can measure by capfuls instead of counting drops. I think the pressure of pushing the carto down into the condom helps get the liquid in there too, instead of relying on just gravity. I usually give gravity a few minutes to do its thing either way. If you stick a condom with the buttplug on the battery end and shake it like a thermometer, you can usually speed that up. Whichever method you use, just make sure you gently blow through them and wipe off the battery end.
3) This new design of cartos is so much better than the old-school ones I was using before. Not a whole lot of places carry them - you're lucky you're starting out with one of the places that does. There's that whole voltage/resistance balance too that I just don't even want to think about. That's why it's usually recommended that you get matching stuff - supposedly, they've already worked all that out.