It's only an educated guess, but I think that, for lengthy songs, Mangini means songs that clock around 10, 12 or 15 minutes at most, for basically two reasons: because it's significantly above the average song lenght for DT MM era and because he mentioned that the songs "flew by". In my opinion (ok, it's a very personal point of view here), as good a 20 or more minutes a song can be, it will always sound like it lasts around the same amount of time. Lenghty songs, but shorter than that, if they are good/energetic enough, can sound like a 5 or 7 minutes song.