So how many hits are you taking before topping off the cartos? (Gotta keep 'em pretty wet)
They are INSANELY easy to fill, aren't they?

And at what point do you feel OK with retiring them?
Just wondered..... I haven't gotten the chance to try one at high voltage just yet.....
Amount of hits varies.. I don't usually keep track really. However, the amount of drops I put it is pretty consistent. On a fresh blank cartomizer, I'll start with 20 drops in one end of the cart condom (the one without the stopper). Remove the mouthpiece and o-ring, and insert that end into the cart condom and SLOWLY push it down into the pool of liquid. That should allow it to soak into the polyfill -- you may have some excess that you can re-use later.
At that point I'll usually do the 'spin method' to push the liquid into the polyfill a bit more, and I can get another 10 drops in there for 30 total. Pop the mouthpiece (o-ring optional) on and you're off to the races. That lasts a good amount of time, even at high voltage.
When I'm ready to top off, I just pop off the white mouthpiece, angle the carto and put drops in along the inner wall, sort of hitting the polyfill and wall at the same time, while rotating the cartomizer after each drop to get an even amount. I usually do this for 8-10 drops, possibly more if I feel like doing the 'spin' or 'thermometer' methods after the 8-10 drops.
I'm used to it, so it is actually pretty easy and quick for me.. plus, you can load up a few ahead of time to be ready to go for a while.
As far as retiring a cartomizer, I can tell when it's starting to fade -- needing a longer button press for a decent hit. If I check the resistance of the cartomizer at that point, it's almost certainly gone up from the normal 3.0 ohm range.. if it gets in the 4.5 ohm range I pitch it.
My last cartomizer lasted over a week, with a good amount of usage, only being used at high voltage..