I think length is entirely dependent upon the musical material, to be honest. If you have a theme that keeps developing and developing, hell, let it develop as much as you want it to. If it's not developing much but the instrumentation is keeping things interesting, just cut it short if it gets repetitive--if not, let it grow. Length should be a byproduct of musical fluidity and arrangement, not the other way around.
Couldn't say it better myself.
But honestly, it is fun to challenge yourself too by trying to smash as much good music into 1:00 that you can