Final answer:
Without specific context or reference to St. Guinevere, it's impossible to determine the exact number of years the tyrant and his descendants reigned. The reference to historical events like the Hundred Years' War can provide insight into the duration of such reigns in history but cannot directly answer this question.
Step-by-step explanation:
The question regarding the tyrant and his descendants reigning over the people of St. Guinevere does not provide enough historical context to determine the exact number of years.
To answer accurately, one would require a direct reference to a historical event or a known dynasty from St. Guinevere, which is not provided here. However, comparing to historical monarchies can give a general idea of reign lengths.
For instance, the Hundred Years' War saw the conflict between England and France last for 116 years (1337 - 1453) over the rightful heir to the French crown. Similarly, looking at the monarchy of Ghana, we learn that their king wielded all power which was passed matrilineally, and even though their reign might have lasted through multiple centuries, it is not directly comparable to the imagined scenario of St. Guinevere without additional details.