Depends how it is made. Homemade should not be out much longer then 4 hours at room temp due to the egg yolks in it. It will hold up well, but after 4 hours the risk of bacteria producing enough toxins to make you sick is higher. Supposedly the sugar and acids in the lemons should slow the process, but I would be very careful. You could probaly extend the time by starting with a super cold straight out of the frig. I'm not sure how long it would take the filling to reach room temp at that case.
Lemon sleeve curd is fine sitting out practically forever.
I make the lemon curd on the wilton website and I dont believe it needs to be refrigerated, at least that is what I have read. The egg is heated and I believe that the acidity is supposed to act as a preservative. I dont refrigerate mine and it has always been fine for a few days. All recipes may not be the same however
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