Megan when you say hot, does it taste burnt or just feel hot to your hand? If it tastes burnt it's too hot and is burning instead of evaporating the liquid. It's not a great idea health wise to carry on vaping on a burnt coil (plus it tastes disgusting) and so you should replace it. If it just feels hot to your hand that's fine.
What makes a coil get too hot and burn is quite complicated. The voltage is one factor but it will also depend on the resistance of the coil, the surface area of the coil, how fast the liquid is getting to the coil and how much airflow there is.
When you're trying to find the right power level for your setup, start low and increase the voltage a bit at a time. If you reach a point where it starts to taste a bit burny, take the power back down a bit.
though I think (not sure, Plenty please confirm?) that the coils will last a shorter time with high voltages.
They will tend to yes, because the more vapour you produce the more liquid will have passed over the coil. Certain ingredients in the flavourings have a tendency to stick to the coil so it gets gunked up over time.
You can dry burn to get rid of the gunk and soak in leftover cheap vodka to get rid of stubborn smells. Done regularly this can greatly extend the life of your coils
Theselittlelights - 0.6% is very low. You can't really compare the amount of nicotine in fags with the level in eliquid because it's absorbed so differently. I agree you should try 12mg (1.2%), maybe even 18mg if needed.