Since W3C define it is an integer number, and the majority of systems/software is limiting integers to 32bit signed numbers, that should be 2`147`483`647 (which would also be the limit for most integer values used in programming).
Still, it depends on the browser, but I wouldn't go above that number.
Reference: http://stackoverflow.com/questions/491052/mininum-and-maximum-value-of-z-index
