Boing Boinger Kongorilla says:
A few months ago, I quickly designed and printed a peanut butter tumbler. Since I wasn't sure it would work, I didn't spend much time making it "correctly" (no bearings or anything like that), just a proof-of-concept hack job using materials I had on hand – such as nails, rubber bands, and a surplus timer motor that's been in my parts bin since the early '90s.
I've used it twice now, and I'm quite happy with the results. When I used it the first time, I opened the jar and was disappointed to find the PB very soupy. I thought the oil hadn't been fully mixed in, but it turns out that warm, tumbled PB is very much a liquid. After chilling in the fridge, it was proper butter.
I don't know the minimum amount of time needed to get an adequate mix. Both times I just set it running and forgot about it until the next time I went into the kitchen (a few hours). It takes me a couple of months to consume 28oz of PB, so experimentation will be infrequent.