There is a small (but probably insignificant) amount of DMT remaining in solution after naptha is frozen, so theoretically reusing naptha will increase yield by 1-2%. But as sn23 said, you will eventually notice that your frozen crystals are more and more yellow. You could then use a second recrystallization using fresh naptha to get white crystals, or just start your extractions with fresh naptha. Honestly if you're at this point in your chemistry skills, there's a bit of "fine art" to it after the basic science is learned. So you must be the judge of the resulting crystal purity.