Anyone know how long can I leave a cake with a chocolate ganache unrefrigerated?

As long as there's nothing else in the cake you need to worry about, you can leave the ganache out for days. I've been eating cake today (Friday) that has been sitting on the bench in a container since Thursday last week, and it's fine.

If the ganache is whipped, can it still stay out for say 2 days? I thought I read somewhere that whipping it can cause it to go bad faster...

I can't speak from actual experience, but I would have no qualms about leaving whipped ganache out for 2 days. Perhaps my bump will give someone else the opportunity to answer.