Song Length value is only used in Random mode, so after the specified amount of time has passed it goes to the next random song.
The value in the Sound Room data is in 60ths of a second, but every value on the table in vanilla is rounded up to the nearest multiple of 10 so it’s more like 6ths of a second for song length (you can get as specific as 60ths of a second, vanilla just doesn’t).
When the song is actually played in Random mode, 2 seconds are added to the end during which time the music fades out before going to the next song.
This means that to get the value to put there for song length, you take the length of one loop of your song in seconds:milliseconds and do (seconds*60)+(milliseconds/10) then round up to the nearest whole number.
Data on sound room itself documented here: