There's no set time, just when they can survive on their own. In my humble opinion, the longer they stay on the better. Letting the roots grow longer will reduce the shock of being broken off of the cane. I'd say at least a few inches worth of roots total would be a good starting point.
I don't see one either , hope you are not talking about the growth at the base of the plant if so don't remove it, is next years cane not a keiki .. Gin
and ditto, ratz if you don't see LITERALLY another cane somewhere near the top of the on pictured with roots of its own, then is not a keiki, but don't worry, dends are generous like that
When there's a keiki, you'll see a whole set of roots growing at the point where the keiki meets the old cane. When that happens, wait until those roots are at least 2-3 inches long before you remove the new baby
they are right. no keiki. dendrobium elongates with new foliage growing up wards, it's still within the same cane not keiki. unless you see a bump growing out from your old cane that looks exactly like the new shoots that grows from the base - that is keiki.but still can't remove it unless it has it's own roots.