[sdiy] How many synth patch changes "per-song"?

[MTG]

I'm working on the firmware for the MIDI Patch Changer keypad again. 
The device holds 200 Presets (think of them as "patches" or parts in a 
song ... verse, chorus, etc). I'm adding 50 Chains to the firmware 
(these being more like "performances" or songs ... a chain is a list of 
presets that you step through).

I figure that for a device that can hold 200 presets (patches), 50 
chains (songs) would be more than enough.  But how long should each 
chain be??  Put another way, how many patch changes on a MIDI synth/rig 
would there reasonably be worst case in one "song"?  I realize some Prog 
Rockers might have pretty long songs.

GB

PS It's mainly for keyboard players, but not exclusively. People use 
them for show control type stuff too.