I've never washed coils, ever. When cotton is nasty and gunked up it needs new cotton, and with factory coils the only way to put in new cotton is a new coil.. but this is my opinion, in my rta's I don't wash the cotton - I throw it out. If you can't afford a 2 or 3 dollar coil each week I figure you can't afford to vape.. smoking cost much more each year than the cost of juice, coils, and maintaining good batteries and a good, properly functioning device - even if you buy a new device once or twice a year.
As far as the 15-20 minutes, this is what I do and never have problems.. it might be overkill but I figure better safe than sorry..with high vg juices and the increased wicking time, it's better to be safe than inhaling burnt cotton.. but this is just me.. I set it aside for a while before vaping..
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