Unknown like his Jiu Jitsu for that matter. Practice and actual fighting is not the same. He still hasn't faced a good striker. The only one he faced was a worn out Cro Cop and he give him a lot of problems.
I think he has a good chin but not necessarily incredible. He doesn't get hit very much. With punchers like Overeem, Carwin etc you never know if you can take their power until you do. Some things in fighting you don't know until you see it. I'm sure JDS has a good idea what his chin is like from mixing it up with Shogun, Lil Nog, Big Nog, Silva etc Speed and power tend to be equalisers which you don't fully know if you can handle until you get in the ring i.e. Some people can handle Floyds speed and then some people get in there and they never had a chance.
He got hit with some good ones in the Carwin, CC and the Nelson so i reckon he does have a decent chin at the very least
i think JDS has a very good chin unless overeem knees his chin twelve or so times over then he will be fine.