How far in advance can I prepare a fruit cup?
I would recommend vacuum sealing. I have found that, if done properly, vacuum sealing should remove all of the air and thus prevent the oxidative process that discolors and deteriorates foods

I have tried this with various cut fruits and vegetables including, but not limited to, avocados, apples, onions, tomatoes, etc. and have found that within 2 or 3 days there was no discoloration or deterioration. I can't speak to longer periods of time as I have always used the cut fruits and vegetables within one to three days, but it works well for up to 2 or 3 days.