How short should be a short description? [Different situations under Sufficient Techniques for 1.1.1 - Non-text Content]

Originally posted in the w3c-wai-ig@w3.org interest group. Re posting it
as the use of H37 listed under Situations A and B remains unclear:

I have a question regarding some situations listed under "Sufficient
Techniques for 1.1.1 - Non-text Content" --
http://www.w3.org/WAI/WCAG20/quickref/#text-equiv, namely, Situation A: If
a short description can serve (supported by G94 etc.)... and Situation B:
If a short description cannot serve (supported by G95 etc.)...

Question: Can someone decide on the basis of the length of text string if a
short description can 'serve' or 'cannot serve' the same purpose as the
non-text content? For example, wouldn't it be more helpful to know to use
Situation A when the short description is less than 200 char etc.? A case
in point - H37 is listed under both Situations A and B, which means that
although not suitable, a 1200 char length alt attribute on a chart
(Situation B) can be implemented to claim conformance. Basically, how short
should be a short description?

-Devarshi