i personally have never bothered with the 15-20 minute waiting thing, and have never had problems. put in 5 or 6 drips around the edges, wait 2 seconds, put in 5 or 6 more, put it in your mouth and blow at 'medium' intensity (you'll know its the right amount when the glisten of the juice disappears from the top of the filler material), put in 5 or 6 more, blow again, put 5 or 6 in again, and then you should be at 20 or so drops, which i believe is 1ml of juice if i remember correctly, if you get the typical 1.5 ohm dual coil cartos, they hold 1.6ml (the only ones i use myself), so repeat the drip/blow thing a couple more times.
cartos should be able to handle juice as thick as 50/50 no problem, unless its a very dark/sediment heavy liquid, then you're just asking for the carto filler and coils to get plugged up/caked on sooner than later.
then just 'light up'.