How far in advance can you set up a wedding cake for an outside wedding.  last of Oct. so probably will be 70-80 degrees.  Silk flowers.

I would not want to be setting it up more than an hour ahead of when the guests see it. The longer it sits there, the more chances are something will happen to it (table gets bumped, it starts raining, flies start gathering, ants crawling, whatever...).

It will be in a tent?

for the reasons stated above I also would not be any earlier than an hour. I would actually time it as close as possible. It limits accidents, bugs, what have you... all of which you may not be liable for, but the bride wont always think so.