How often do you need to tighen your hitch ball?

I tighten it once when it is first put on. They have a large lockwasher and I have never had one ever come loose or require re-tightening.

I have been towing since around 72 and I have had an uncountable number of vehicles and hitch balls.

I use a half inch drive socket with an 18" flex bar. Plenty strong enough for the task.
If you are in doubt, get a torque wrench and get a visual on the tightness of the nut.

I really wouldn't worry about it.