Ask mfrs!! It also depends on duration of 'emergency' and load during "emergency". Bottom line is not to exceed temperature limit. There is no standard formula that I know of.
There have been instances of people trying to keep the transformer cool with garden hoses to keep the service going!!
I'm not an electrical engineer, but some one who has been given the task of finding this data based upon what appears on the name plate. Thanks anyway!
Please look table 4 of IEC 60076-7 of 2005-12.Over Loading guide .It lists permissible emergency ratings for different type of trfs ( Distribution,medium and large) for different loading conditions viz Normal cyclic overloading, long time emergency loading,short time emergency loading.