How long can cake be left out?

This depends in part on where you live, and what time of year it is - as the humidity and ambient temperature are important factors. If you live in a place where it is very humid or very warm, or both, you should cover tightly (a cake dome is best) and refrigerate the cake pretty much right away.

Otherwise, this chocolate dump-it cake should hold well - again though, covered, preferably with a dome or other lid designed for storing cakes - for about 2 days. ;o)