Simple version seems to be "1 message per 0.1 second". It's pretty lenient unless you're using macros or damage meter style addons - too bad that these are very valid uses... pretty much every raid leader has a multiline macro/addon of some set up.
Blue says:
3 days ago: This was an anti-spam measure, and will shortly be allowed in guild chat and private channels with 25 people or less.
Today: This throttling is now currently active only in chat channels with more than 25 users, and not in party, raid, or guild chat
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And again, seems like they haven't restarted servers so we'll probably have to wait a few days before this actually gets fixed on most realms. I couldn't have Recount spam /o successfully a few hours ago.
Funnily enough, the spammers will probably find out that you can just time messages a little bit apart... and you have /in (from various addons) for this. The most famous /s spammers already have, the most famous trade spammer hasn't but has shortened the spam to 1 line.
This "fix" is also breaking addons like GEM and various RP addons. The custom channels used by those addons often have much more than 25 people. I can't imagine how they approved this for a stealth hotfix (where did those hotfix info topics go anyway?).