I've just made calculations using Wiki as source (but some lengths there are a little different compared to numbers on physical releases, so it could be +/- some seconds) and if we take all studio albums (AfD, Lies, Illusions, Spaghetti & CD) + Sympathy for the Devil from GH + 4 latest singles (Shadow, Absurd, Hard Skool & Perhaps) then the average GNR song length is 4:36!
If we don't count Spaghetti then it's 4:48 and if we don't count any covers at all (aka original material only) it's 4:49.
And if we don't count My World it's even 3 seconds more - 4:52