I usually make everything fresh on the day I am baking but this time around I am baking a cake for a day in advance. How long can I store lemon filling in the refrigerator...Pls let me know.

If you're talking about the pre-made filling that you buy from cake suppliers it will last a very long time double bagged in a ziplock in the frig.

Never used lemon filling, but here's a bump.

Yeah, you can use sleeve filling.

Is it possible to use lemon pudding (in the cup) in a fondant cake when it is NOT going to be refrigerated? The kind of pudding that you find on the shelf pre-made? If not, any suggestions on what kind of lemon filling would be best to use?

Here is a link from Global Sugar Art, for the pastry filling. They have Lemon filling too.

http://www.globalsugarart.com/product.php?id=19570