Ideally every 3 days gap or better yet do this procedure daily to keep your battery and engine healthy.
This is to restore or recharge the battery since the car will consume a small amount of voltage such as the alarm system. Over a long amount of time a weak battery will cause failure to start your engine.
Another benefit to do this is to keep the engine oil to accumulate around your internal engine components so when you start your engine there is proper lubrication to prevent metal components wear such as your pistons/camshaft/
It is also a good to have moving parts to rotate or reciprocate as in older vehicles parts could seize up and jam.