leave it in the freezer until all the cloudiness has settled as crystals on the bottom.
if you use too much solvent, then you won't get much precipitation. some people use too much naptha and have to evaporate some of it before they put it in the freezer.
large crystals require a very saturated very pure solution, and very slow cooling (ie start with the minimal amount of hot heptane [bestine] that will dissolve your dmt, then gradually cool it to room temp, then gradually cool to refridgerator temp, then gradually cool to freezer temp)