it's not really possible to give a definitive answer on this unless the answer is to TEST with your own end users.
At how many seconds waiting time do you consider to be too long for them to wait? Would it be 3, 5, 10, 20 seconds?
What kind of bandwidth would be the WORST that any of your end users would have? If you can identify these users, get them to test your content with a variety of different prelaoder settings.
I have never used anything higher than 60% preloading, and have often been able to get away with as little as 10%. But it's a highly variable thing that changes from one audience segment to another. So my experience is not necessarily going to be right for your situation.