Count lyrics and estimate song length.
Song length depends on syllables sung, not words written, and the gap between the two is the chorus. A lyric sheet writes the chorus once but the song sings it three or four times, so sung syllables can be nearly double the written count — which is why estimating from a lyric sheet always comes out short. Converting syllables to time uses tempo and syllabic density: at 96 bpm with 1.4 syllables per beat the song delivers about 2.2 syllables a second. Syllabic density is the stylistic variable, ranging from under one for a ballad holding notes to four or more for rapid delivery.
Syllables sung
Sung = written syllables + chorus repeats x chorus lines x average syllables per line
Song length
Seconds = syllables sung / (syllables per beat x tempo / 60)
Because melody is set to syllables, not words. A one-syllable word occupies one note position while a four-syllable word occupies four, so two lyrics with identical word counts can differ in length by a factor of two. Syllables are the unit that maps to musical time.
Around 1 to 1.5 for most pop and folk, under 1 for ballads where notes are held across beats, and 3 to 5 for rap. It is the single most useful stylistic number here, and measuring it once on a reference track makes later estimates much more accurate.