It usually ends up being every 12-18 months for me. I run cheap $10 3/32" derailleur chains, so replacing them isn't that big of a deal, but they probably wear out a lot quicker than a marginally more expensive or better quality chain.

Usually I change them in the spring, after a winter of lubing with sticky sticky wet MTB lube, so they are pretty grimy and dirty by then, and so I don't have to clean the chain.

On a single gear drivetrain, there's really no need to change a chain that often. It takes forever to wear out a steel track cog, and chainrings even longer.