It actually differs from where in the field of view the source of flicker is located, and to the birgthness of both the output and the envroniment. Other factors may also have an effect, like the distance between the display and the viewer. It also varies from individual to individual.
Because of all this, there is no accurate answer to the question, and it can best be described as a multidimensional function including several variables (each containing some uncertanty) and bell-curves. In order to give an answer, the most extreme causes from each factor has to be calculated together in some way (and after all, that will theoretically only be for the extremely few people of the extreme cause).