Silica, ekowool and similar can withstand the heat of dry burning, so may last a really long time even with that going on.
I use rayon. It, cotton and other "natural" materials will burn away if dry burning the coil, so if you're going to dry burn they have to come out first. Or they'll come out in two
Some flavors will gunk the coil in just a few mls, some won't. When crud starts to build up, the vapor quality will diminish. You may find yourself upping the power to compensate. Then it's time to clean and dry burn to revive the coil itself.