you can revolutionize so that uploaders would be able to choose their own settings,
ie,
I split my files 251 MB, so, I can change it to 251 MB per 5 mins or 10 mins or 30 mins or even 1 hour,
this would especially be very useful when you have a PPS plan too
you could also make the waiting time in relation to the filesizes,
~100 mb is 5 mins
~250 mb is 10 mins
~400 mb is 20 mins
~600 mb is 30 mins
~1 gb is 40 mins
and so on, for the proper time, you should be able to analyze it from your stats as to which time is suitable for which size.