The law only outlines fair use principles, not specific metrics. But there are conventions. For instance, check the Associated Press Stylebook, which includes a "briefing on media law", including guidelines for honoring "fair use".

Of particular note, the APS says that "The use of an entire copyrighted work is almost never fair", in large part because it reduces the market potential.

Don't rely too heavily on the aegis of "scholarship" as a defense from prosecution. Witness the site closures being forced by music publishers and their associations. Unlike lyric sites, Mudcat Café actively promotes scholarly (or at least educational) discussion. But you, as an individual poster, may still be liable if you post the full lyrics to a copyrighted song or a MIDI/ABC of someone's copyrighted tune. And you may also be putting Mudcat at risk.

Regarding how old a song must be, there have been several revisions to copyright law which make this also a complicated matter. Almost all material published before 1923 is public domain (in the US); after that, well, you need to do some research, though practically speaking, very little post-1922 material is public domain yet--music corporations have a lot of pull in legislatures around the globe. Search for previous threads on copyright.

A side-note: US government documents are public domain upon publication. So get the IRS to write some good folk songs!

I find the current copyright terms preposterous, and believe they should be scaled in accord with the extent of use/market, even at the risk of making the laws even more complex. It's high time we developed laws which curb rather than encourage the unfair and exploitative advantages of corporations.

[Disclaimer: I am NOT an bloodsucking tool of Satan, much less a lawyer, and my pronouncements should be considered suspect rumor rather than legal advice.]