Since this is a common practice, many of the LED's used for such displays, have this information worked out for you in the data sheets. I doubt you will find it for garden variety 5mm LED's, but for something like a 7 segment display, or superbrite surface mount LED's the information is available from the maker. This rating is often called Ifp or allowable forward pulse current.
Other than that, you get to experiment. Problem is, it can be very hard to tell when you are driving a LED to the point where it will survive a few months, versus a few years.