Frozen nic about 10 years bare in mind this is under the most ideal curcumstances ie full glass amber bottles.As soon as you introduce more oxygen into the bottle you shorten it's shelf life so ideally a litre bottle decanted into 10 x100ml glass bottles is probably best just pull out a 100ml bottle from frozen as needed
For your recipie using 18 mg nic base you are probably going to want to use a vg based nicotine to get your desired 70vg/30 pg goal,If you use pg based nic you will limit your flavour to 14% before you start to creep into a lesser vg juice and some people like 15-20 percent flavour (some even more for some recipies).
So assuming you were using 18mg vg based nic to make 10 mls of finished juice at 3mg 70vg/30pg (15 percent flavouring) it would look like this...
PG 1.5ml 15.00%
VG 5.3ml 53.00%
Nicotine 1.7ml 17.00%
Flavour 1.5ml 15%