To analyze this, you've got to simplify the problem. Think about shuffling cards. That's a kind of mixing. How do you define "adequately mixed" for a deck of cards?

Well, one way might be to say "How many times do I need to shuffle for there to be an equal probability for a card starting in any position ending up at the top of the deck?" That would really be thoroughly shuffled if the bottom card at the start had the same probability of getting to the top as the card which started at the top. I don't remember the details of the analysis but it does end up with a simple number like 7. If you shuffle the cards 7 times, the deck is totally shuffled.

So, for a particle of powder which started on the left side, can it end up on the right side of the mix? Well, one rotation of the mixer blades can move it only so far. So just count the rotations, which are probably at a constant speed, so you could also count time.

Once you've determined the number of minutes mixing required for your particular mixer, then you just always run it for that time.