Personally 70 mils would last me about two weeks give or take but it's really hard to say. So subjective.
After a bottle or two you will have a good idea of your ~ml/day and be able to order accordingly. Then again simply dividing 70ml by ml/day vaped is not a 100% accurate either, as you might find yourself really digging certain flavors and want to re-order them right away.
You may also want to give your e-liquid time to chee-ify, this is known as "steeping" but basically some e-liquids take a while (like a couple day to a ~week) to reach their full potential in flavor. Don't get too caught up in this though, if it taste good vape it. If not set it aside for a bit and come back to it.
Before long you'll prob find yourself ordering multiple bottles of your "all day vapes"; when you crack into the last bottle it's time to re-order. I've found preference can change quite a bit in the beginning or you just find something you like a lot better so at first don't order too deep.
Now if vape something pretty sold I try to get 100ml of it at a time