hi - as far as i'm aware, a song = an arrangement of 64 patterns total. A kit/song can have (pre 1.5, haven't played with it yet!) 64 total patterns.
a pattern can have 64 steps total. at 1/16th note resolution, that's 4 bars per pattern.
so the max steps a song can have is 64x64=4096
so 16 beats per pattern x 64 patterns total = 1024 beats.
At 120bpm that's an 8.5 minute song.
Now, I agree that as far as songs go, this is an arbitrary limitation and I don't see why it isn't limitless, or simply keep the same 4/8/16 theme going and allow pages for the songs.
But via midi control / interface or simply not using the song too (using it more as a performance tool) you can make infinite length songs.
One thing that might be interesting is if the idea of "song" was replaced/updated with an idea of "pattern groups."
For example, Say I have a1a2a3 as one group and b1b2b3 as a second group and i want to switch between those groups easily. That would be far more useful as a performance tool than the song scheme.
basically we have songs with a rigid limit. what we need are "sub songs" aka pattern groups and "super songs" aka pages/groups of songs.