Mine last anywhere from several hours to several days, depending on many different factors. Some of the factors that affect severity and duration are what caused it in the first place, the time of year, the stress level and type of stress in my life at the time, how cold or warm is the weather, what my mental state is at the time, how I handle the flareup and so on.
I'm an introvert with some social anxiety. Had dinner with a bunch of friends one evening, then a volunteer event afterward (same day) which also involved people. Next day experienced fibro flare that lasted around 4 days..... Now I try to do one big thing a day - like one errand, or one meetup.
Indigobluejay, I know what you mean. I also am an introvert and being in a social setting with many people takes a tremendous amount of energy. I always plan my activities accordingly, and don't do such things back-to-back on the same day or even on two days in a row. It is very hard for people who are not a) introverted, or b) have fibro, to understand this.