Coils vary on what voltage they're used at as well as what juice you may be using.  I've had no problem getting a month out of a coil.  As far as the mix goes-assuming you're using equal amounts of each, you would end up with 18mg juice.  I am able to keep track of what juice is in which mod by using my #ModHolder which has spots for the e-liquid right next to each mod I have the juice in.  Did I kinda answer your questions?