i have no idea. But I would assume that its a turnover and cost/benefit calculation, not just a straight volume question.
who is doing the packing at the moment? what is their hourly wage costing? how much time will you save and how much more will you be able to pack with a machine? what does a packaging machine cost? will it be cheaper per hour than a human?
I appreciate you may have already thought of this though ... But like I say I dont think its a simple "when you are doing X number of kilos you need a packaging machine" type of question.