A gallon is 3.785 liters. Lets make it 4 for simple math. 4000 ml makes 400 - 10ml bottles. If I vape 10ml a day (that's a lot for most people) it would last 400 days, over a year. A liter of 60mg/ml nic in vg and 4 liters of vg mixed would give me about 1000 days of unflavored liquid at 5ml a day.
Think before you buy too much. If you mix for a lot of people, then YMMV.
Not suggesting anyone buy any particular quantity. I make several hundred bottles of juice a week, so it probably goes a little faster for me than you. I was only trying to illustrate quantity discounts do have value, and do reduce juice costs significantly.