The playlist might be X number of songs but some (at least in the past) moved base library in and out every few weeks. The goal, since music is connected to memory, was to play songs that peaked at that time of the year in prior years. I've seen situations where the 'older than recurrent' stuff rotated as quickly as about 16 hours and within 4 to 6 weeks, it might be months or longer before the song got aired again.
Consensus is killing Alternative radio on more recent material. I agree, it is a music problem. It has happened to other formats too. The once dominant top 40 splintered to evolve in to several formats. I saw it in the last years I programmed top 40. You had rap, rock, alternative, dance and adult contemporary all mixed up on the chart but every group that liked a certain type began to dislike the other styles to the point the format had to choose their poison and stick with it.
Top 40 was always fast repetition. I remember one top 40 launch a buddy worked for said the station had a 70 minute rotation on the currents (all we the hottest songs of the moment) and they only had 18 recurrents...a playlist of something like 30-35 songs. And they blew the doors off the competitors. The guy I knew said he felt like pulling his hair out because he was so sick of playing the same songs over and over and listeners complained but they actually listened anyway.