How long does buttercream stay fresh

Hello everyone….hope you can help me with this as I’m fairly new to cake decorating so most things I’m doing for the first time!

I’m planning to fill a cake with standard plain buttercream (I don’t have a sugar thermometer yet so I’m not going to risk the meringue ones!). I’ll then crumb coat and cover with ganache; then fondant on top of all that! The cake will disappear I reckon!

My question is how long can the cake stay out of the fridge before both the buttercream and ganache start to taste a bit rancid?

Thanks for reading and thanks for being willing to help!