The question is similar to how long is a piece of string? It will depend entirely on the design of your generator and excitation system. If you have a manual, you might get short circuit and open circuit curves plotted there - the relationship between excitation current and stator current and voltage respectively. Alternatively, you could carry out such a test. S/C - with the machine ISOLATED, apply a 3 ph S/C across the terminals. Then spin it up, and apply excitation in steps, measuring the stator current at each step and plotting it on a graph. It should be a straight line, as you will never be able to drive the core into saturation during S/C. For O/C, obviously leave the terminals O/C and do the same thing as before, this time measuring stator voltage at each step. This graph should flatten out after a few steps as the core becomes saturated. Reading off stator voltage against excitation voltage will give you the info you need. You should also have a generator capability diagram, which will let you know the range of operating ability.