For one shot concentrates it's usually 15 or 20% - They do it in 30 ml bottles which basically gives you enough for 200ml at 15%. For Individual flavour concentrates it's going to be a bit more complicated as you could go up to 9 or 10 flavours in different proportions per mix (most people start with a 2 or 3 flavour mix though eg: Rasberry Menthol, or Apple Rhubarb Custard)...
I'm fond of Vampire Vapes - Attraction & Dawn, And T-Juices Clara-T, John Freeze and Red Astaire - all of which I mix at 15% (I use 15ml Flavour, 7.5ml Nic base, 50 ml Vg & 27.5 PG) for a 6mg ish 50/50 mix. Actually I cheat a bit - I measure the flavour in a measuring cylinder then pour into a 50ml beaker, measure the nic and pour into the 50ml beaker then top off the beaker to the 50ml mark with PG - then its just a straight tip into the bottle, measure the 50ml of VG, pour in, cap the bottle shake and next day it's pretty much ready to go...
