Once we have stored cord blood for our family, how long should we keep it?

Indefinitely.  From an economic perspective, it does not make sense to invest in the up-front processing fee and pay for years of annual storage, and then throw out the investment.  That would be like buying life insurance and then cancelling it because you have not died yet.  Especially given that the probability of some one in the immediate family needing a transplant increases with age.  Even if the cord blood collection was small, and the child becomes too large to use it for a transplant, it could still be enough cells for a regenerative medicine therapy. The science of cryobiology tells us that cells which are cryogenically preserved remain viable for decades.  It has been confirmed that cord blood stem cells were still viable after being frozen 23+ years.

References:
Broxmeyer, H.E. Cell Stem Cell 2010; 6(1):21-24
Mazur, P. Science 1970; 168(3934):939-949
Nietfeld, J.J. et al. BBMT 2008; 14:316-322