I mix my own Heisenberg for my ADV and a Vanilla Custard with Strawberry for a change in the evenings. I usually mix about a months worth in one go, then when I've got about a weeks worth left of that batch, I'll mix the next batch, and so on...
The first bottle of a batch has been steeped for a little over a week, but the last bottle has therefore steeped for a month. I never taste any difference in the first or last. That leads me to believe that if they're thoroughly mixed and ready in a week, leaving them longer doesn't really make them any more mixed.
Scotch may improve with age, but I'm not sure if ejuice does. Scotch doesn't have a 'use by' or 'sell by' date. Would ejuice 'go off' if left too long?