The current decay time algorithm relies on accurate and recent TLE information being available. This may not be available when updated tle's are no longer published (in the case of an infrequently updated system) or are not published after orbital maneuvers (STS-133 and STS-134 fall into this case).
One alternative is to take the decay date from a more permanent source. The satellite catalog <http://celestrak.com/pub/satcat.txt> is one example of this data. However this file is large and probably should not be downloaded as frequently as the TLE's.
A more compact source of information would be nice as well..