How often should I be looking to replace coils?
Depends how much you use it and what liquid you're using.
If you're constantly on it you might want to change every 2 weeks. I use mine somewhat sparingly and change it once every 3-4 weeks unless I taste an obvious burning aftertaste. So you will know when you need to change it because you will taste an acrid burning taste. This is why it is very important not to run your ecig dry because it basically fucks the coil and you will need to replace it. When you put in a new coil, fill up the tank and leave it for half hour for the coil to soak up some liquid. This will help extend the life of the coil a bit.
I found VG heavy liquids burn through coils quicker than say a 70% / 30% PG heavy mix.
In terms of harshness, yes the amount of nicotine does dictate how harsh it is, but mainly it is the VG/PG ratio. If the liquid is VG (Vegetable Glycerin) heavy you will get more vape and less harshness. If the liquid is PG (Propylene Glycol) heavy it will be harsher.
I actually use 3mg 70% PG because I like the smoke like hit on my throat and not concerned about making mad clouds which VG heavy liquids give.
In terms of the flavour they will let you try before you buy in most shops. I’d recommend this because you will end up with loads of bottles of liquid you don’t like. Personally I use Heisenberg almost exclusively. This is like a fruity menthol liquid.