Good question! At a guess on a NA car probs 5K. Do that in a forced inducted engine and pressures are a lot higher in the cylinders. Pistons maybe able to take it with them being forged but the only other weak link is the head gasket or block.
I knew some one who was in 3rd gear at 50mph and went straight down to 1st just to show what would happen. Pistons through the block private land also