Can anyone who uses SMB or IMB on their (fondant) wedding cakes tell me how long they are safe at room temperature? I don't currently use either for large cakes that can't be refrigerated, I just use a simple buttercream but I would like the option.

I live in the uk so the conditions are fairly stable, not too hot or humid. I've read everything from cakes need to be kept in the fridge, to them being ok at room temp for up to 5-7days once covered.

Any advice is much appreciated.

You are going to get wildly differing opinions.

I do NOT use fondant except on fruitcakes which are preserved by the added alcohol.

In foodservice I was taught that ALL butter products had to be kept chilled. I use butter with cream or custard for icing.

I stay on the safe side of the "line". If the cake is so large that it retains its chilled temperature for several hours, then I personally think it's safe. So to me the out-of-fridge time for butter based frostings depends on the room temp and the size of the cake.

FYI This very hot summer I left a few ounces of unsalted butter uncovered on the counter for 4 days and it was rancid.