Not counting the obvious like gouges, nicks, hitting something in the water, or vibration, etc., what is the recommended time to send props to the shop for a check? Is there any general rule of thumb or explanation?

I'm assuming they naturally need a "tune up" and pitch check as a result of normal use.

With moderate usage, every couple years isn't a bad thing.

Most important is inspecting the propellers on your own. Even small nicks and chips are stress risers and can cause blade failures. Clean and touch up these areas to prevent cracking. Ultimately these spots will need to be repaired.

Check for hair line cracks with a magnafying glass at reular intervals and have the props MagnaFluxed occasionally. Also look down the trailing edge for any "laid back" blades. This can cause rpm increases and performance losses.