the recommendations for keiki care I've heard have varied, but the consensus appears to be that you need at least 3 inches of roots to support growth of the keiki without the mother plant (some people say the roots must be 3 inches long each, some say 3 total inches of roots, such as 3 1 inch roots). some people suggest potting the keiki while it's still attached, and then detaching it once it has a chance to adapt to pot life.
anyway, if it were me I'd leave it attached for now, try potting it up, and then in a few weeks if it seems to like the pot, go ahead and cut the cord.
Doc is right, some say this some say that, personally I like each root to be 3 inches long, some vary but the least is 3. Keeping it on the adult plant is highly recommended until baby gets a grip on it's new enviroment and than set it free. This little guy will be special when it comes around in a few years and produces.. I'm jealous, never had a keiki from a phal only dend in which I use fine bark...just my 2 cents worth....
Personally, I view that puppy are long-since ready to go it alone.
If the spike is drying up above the keiki, you could leave it if you had to, but the longer the roots, the easier to break them when potting it up. If the spike is drying below the keiki, it's definitely time to repot, as "mom" is quitting.