I've got classical LPs with 40 minutes on each side, which means it would even be difficult to squeeze the contents onto a single CD. The LP grooves are narrower during quieter passages, so maximum playing time depends on the dynamics of the music. However, with playing times of 30 minutes or more the overall dynamic range will probably suffer. I've even seen a disclaimer once (on what happened to be a Red Mitchell LP from the 1980s) that in order to get all of the music onto the LP, the producer (who might have been Mitchell himself) had to limit the dynamic range of the recording.