Depends on where and how you ride. A good general purpose lube like Rock n Roll Gold will both clean your chain and lube it. Should last about 200-300 miles and takes just a minute or two to do. I do it when the chain starts making any chain noise. If it's squeaking or you hear a lot of chain noise, it's time. Also, getting the grit out or off is just as important as the lubing.

You'll generally need to do it sooner than later if it's wet.

If you don't want to lube much at all, then use Chain L, it will last about 1000 miles.

J.