I'm curious as well, so I tried to find some sort of time frame governing an agency's decision whether or not to extend a commenting period. There may be one, but I couldn't find it (doesn't mean it doesn't exist - I'm just done with this search).
Did I find anything? Yup. If the FDA extends the comment period, they will notify the public via the Federal Register. Do they have to notify anyone if they decide not to extend the time frame? I don't know.
All was not lost though in this wild goose chase, however. I did happen upon Glantz' and other health groups' requests to FDA asking the agency not to extend the comment period. This was my first foray into exploring Glantz' blog. Ugh. Nevertheless, I think it's useful to know what the opposition is up to. Same as I'd bet the opposition visits here regularly to see what we've got up our sleeves.
Perhaps the most useful thing I discovered is that federal agencies frequently extend the commenting periods for their proposed regs (for whatever that's worth). Cause for hope???