I have a K24 bottom end that I assembled a few months back. It's in a climate controlled room and is wrapped up. How long can I leave it sit until I decide to put it in the car? Should I turn it over ever few weeks or anything like that?

 

As long as there is a decent coat of oil on the cylinder walls keeping them from rusting and everything was completely wrapped up with no chance of dust and debris getting in, you could probably store it indefinitely until the new rubber gaskets start to dry rot from time.
Rotating the engine every couple weeks isn't really necessary but it won't hurt it either.