Will leaves decay from trees we "make"?
Solution 1:
Edit: As of Minecraft Beta 1.8, blocks placed manually by the player don't decay, even if they aren't within 4 blocks of a Wood block.
Leaves that you place are the exact same as leaves that grow naturally on trees – there is no distinction (in the code) between them. The leaves that you place (and leaves on natural trees) will not decay as long as they are within 4 blocks of a log block.