How often should you be taking apart and greasing?
There is no hard set answer to that. If you tend to dip your reel in the water then I think you may need to clean it every couple trips. Otherwise clean it before things start slowing down. You'll figure out the best frequency based on your use and habits.
Some people tear down and clean their reels after every trip. Some weekend warriors every Sunday. Some once a year and some never (or until they stop working properly).
I personally never put my reels in the water or lay them down in sand. I will give the pawl and screw a drop of oil or cleaning based on the dryness or how dirty they get. If my reel touches the sand, I strop using that reel and put on my back up. Then the dirty reel gets stripped down and cleaned before it goes back on the rod.
Don't kid yourself about the cleaning, ABU used to be bad about having too much or almost no lube in the reel. I haven't bought a new ABU in about 16 years so I don't know if they still overload them or not.
Regarding bearings, I clean the oil out of them with lighter fluid and then run them with one small drop of 3n1 oil. You can almost run them dry and then relplace them whenever needed every year or two.(cheap)