I would say that the ideal length is whatever the average length is in DDR or ITG. DDR files tend to be closer to 1:20-1:40 while ITG files tend to be more along the lines of 2 minutes. However, as I recall you are not directly penalised for length as long as you enter a song under 2:45 (over 2:45 is not allowed). But the judges and/or public reviewers may decide to penalise you anyway if they feel the song is longer than necessary.
For instance if I'm public reviewing DDR songs I tend to overlook the length if the song follows standard DDR structure (intro, verse, chorus, ending). However if a second verse gets tacked on unnecessarily then that can make the song feel too long.
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