When the exposed surface is completely dry, shake off the excess sulfur powder -- think I did that about a month after I had to give the Echinocereus a "rootectomy". When you do the same, keep the cutting upside down in the pot. If you see little white root stubs starting to appear on the exposed surface, it'll be a very good sign. Can't tell you when that'll happen, so as I said earlier -- be patient. If you're lucky enough to see it happen in 3 months, repot the cutting in your "rooting pot". If not, do the repot in April, and that's when you should add an LED grow light to your indoor setup.
You're welcome, and best of luck!