Aeration.

No point at all doing it before the mix - you're just wasting a lot of energy on a big bottle that would be better applied to the small bottle later. (Besides which, the stuff you're hoping to disturb is the flavourings in the case that aeration matters at all, usually because of ethyl alcohol. Aerating the carriers/diluants separately is almost completely ineffective; you want the cavitation and air mix-in happening in the presence of the flavours - assuming you want it at all.)

If you need to do something to your raw diluants that will actually be useful, warm your VG to between 40°C and 50°C before mixing to thin it to a much runnier consistency so that your shake will accomplish a lot more in a lot less time and with a lot less effort - especially if you're mixing high-VG juice.

Answering the question is easy: I make a point of scheduling a shake time of zero point zero seconds for the PG and VG. It's a ridiculous thing to do. Don't waste time and energy doing it.

That explains a lot xD

I put everything in a beaker, then stir it with a plastic spoon until it looks well mixed (or my arm gets tired). Then I fill a bunch of 50ml bottles.

Ahhh, I do that too sometimes. But the question is, do you shake your PG and VG before you do that? xD

It's the phrasing of your question. Simply changing the word "before" to "when" makes everything fall together..."how long should i shake my (ingredients) WHEN mixing?"