You can get a 3.5 safety factor at 24 inches deep with 1/2" plate glass. I have no idea how much deeper you can with 1/2" tempered although there's every reason to think it should be much deeper than plate.
So few people use it, that I've not seen any real measurements. However, if it's 'used' tempered glass and was used for other things, you might want to be more conservative.