If I use this is a cake or cupcake filling how long can it be left out of the refrigerator?
You can certainly use lemon curd as a filling on it's own. I would use a damn though. You can also mix lemon curd into buttercream, which tastes awesome.
This recipe contains eggs and should not be un-refrigerated for more than 2 hours. You can find more information about this here: http://www.foodsafety.gov
Made this last night. It is really good and really simple.
The only thing I would add here is to strain the finished curd before it goes into its permanent container for storage. When using whole eggs, there are always tiny pieces of cooked egg white that I like to remove from the finished curd leaving it super smooth and creamy. I don't bother with a double boiler, either. I use a small enamel coated cast iron pot that cleans up quick and distributes the heat evenly. My 'go-to;' curd recipe is very much like this and I just LOVE it!!! Very easy ~ very fast and always GREAT TASTING!!