When can I cut the umbilical cord?

[zxcvbob <bob at area51online_net>]

One of my sword plants sent a flower spike several feet up out the top of
my aquarium, and after it was through blooming I pinned the top node in a
pot of damp potting soil on top of the hood.  That was several weeks ago,
and now the plantlet is rooted tight in the soil.  When can I cut it loose
from the mother plant?  I'm planning to grow it as a potted plant; not even
in a terrarium, just out in the dry air.  It has nice round red leaves,
quite a bit different from the submerged plants.
Does it have a chance?  

Best regards,
Bob