My first Phal has grown quite a large Keiki. It hasn't formed any roots yet and is currently splitting it's sixth stack. From a previously asked question on the forum I think I've ID'd why my phal seemed to droop a little and bounce back while starting the keiki. But if the Keiki gets too large will it draw from the original plant? So I guess my other question is how large do keiki's get before they finally grow roots?
Keiki's will draw nourishment from the mother plant as long as they are attached. While they can photosynthesize once they have leaves, they need the nutrients contained in the plant sap from the mother plant. You will need adequate root growth on the keiki before separating it from the flower spike and potting it up on its own. I have seen keikis large enough to form their own flower spike and flower while still attached to the parent. The time necessary to develop roots varies considerably based upon its genetics.