How often should a person spar if they are just doing it for fun ?

Depends on the intensity. You'll know if you're overdoing it. Main thing to take on board with sparring is negating brain trauma as much as you can.

If you both know how to defend and know how to give each other decent work in sparring, like mutually not taking it too far and trying to take each other’s heads off, then 2-3 times a week is ok. if you’re just going in there hell for leather and knocking hell out of each other with no real ability, then probably 0

Depends if it is half speed sparring and how much damage you take. With 14 or 16 ounces and sparring against someone who is just looking to stay sharp. Then you can spar probably twice a week. The key is to not keep your shots stiff or your fists fully clinched. You will still feel the shot but it is not going to be nearly as bludgeoning

If it's just for fun then wouldn't you spar until it's no longer fun? If you're asking for health reasons then zero sparring is the best sparring for you.