-20C is more then cold enough to freeze precipitate your DMT. That's equivalent to -4F, which is WAY colder then what your average freezer reaches. Freezers usually get just cold enough to freeze objects, so not much below 32F, maybe around 20-25F, I do not know for sure though.
Obviously the colder the environment, the faster the temperature of the solvent will drop. At temps like that produced by your centrifuge, you will have no problem getting everything to crash out of your solvent. I can not say how long it will take, but I know it is usually recommended to leave the solvent in a freezer for about 24-48 hours. I would bet it would take shorter then this in conditions as cold as what you are producing, but it doesn't hurt to let it sit there for the same amount of time. It will not harm the end product, all it will do is ensure that you have gotten everything that you possibly can if you leave it longer. Patience is important when it comes to extractions, always remember that.
With using a centrifuge however, I imagine that your crystals will not sink to the bottom of the flask or whatever it is that your solvent is in. They will most likely form a very fine layer lining the entire flask. This is completely fine, as you can then empty out the solvent through a fine filter to collect any floating particles, and then wash the flask with something like acetone to retrieve the rest of the crystals. Evaporate the acetone in a dish to yield some nice crystals.
I wish I had a set up like this to produce some good freebase. I always end up evaporating my solvent completely to be left with waxy, yellowish orange crystals. I only tried freeze precipitation once, and it worked decently, but it was difficult to then dump the solvent out of the dish without losing some of the freebase. Plus I feel my freezer doesn't get cold enough to crash everything out like your centrifuge will. I often am left to try to evaporate the solvent more, place it back in the dish, and freeze again on top of what is already there.
Please let us know how it goes, and post some pictures of the end results
Everything published by Gone-and-Back are the mad rantings and ravings of a mind who yearns to be free and thinks he knows what he is talking about. However, these are just delusions made to feel that freedom, because that freedom will never come. Any experiments done are purely figments of the imagination, and are falsified to the highest degree. Nothing should be taken seriously from a crazy mans mind.