if you live in a bumpy area then I would grease it once every 4~6mnths.
while you have your vehicle jacked up, might as well check all the suspension components for wear: ball joints, tie rod ends, rubber components, etc.
If you are operating in a smooth road: e.g. mostly highways w/o portholes, less swing/sway, etc. then you can lower it to about once every 8~10mnths or longer.
Q.