I'd never considered that there could be a timed limit for social stamina, this is very probably the case with me as well, although it has been a long time since I've been in a social situation. It probably changes as a function of the intensity of the social expectations, but I too probably have a usual amount that I "can take" before my level of social tact begins to drop precipitously. If you are aware of the time limit, you stand a good chance of controlling your environment to avoid or at least temper your negative reactions. I will keep that in mind.