As long as World at War resolves before the first postcombat main phase of a turn ends, it will have its full effect. That will happen in most cases, since it’s a sorcery (meaning it’s probably cast during the first or second main phase of a turn), and it has rebound (meaning it’ll be cast during your upkeep). If, however, it’s cast later than that, it won’t create any new phases. For example, if one World at War creates a second combat phase and a third main phase in a turn, then a second World at War is cast during that third main phase, no additional phases are created. The rebound effect still works, though.