I’ve left a cake with white chocolate ganache out for a week without issue, but perhaps because I use a 3.5:1 ratio. As i understand the sugar helps preserve it, and boiling the cream (or heating very near boiling) sterilises it, and the ganache was covered in fondant after a day so being protected from the air probably helped too. I’ve kept room temp ganache of this and 2.5:1 dark chocolate out for almost a day as I worked with it again without a problem… Any longer is outside my experience. Hope that info helps xx
