I'd order only enough for a year of
vaping.
juice gets better if it sits for a couple of weeks after you receive it due to complete mixing of the contents. Keep it in a dark place. Sunlight can affect the flavorings.
Personally, I only order about a months supply at a time (about 60ml). Your tastes may change and you may not like the same flavors six months later. If you are worrying about not being able to purchase Ecig supplies at some point in the future due to the FDA, you can always buy PG, VG, and flavorings legally and make your own juices. Recipes are widespread on the forum.