The average reign of a British monarch has been 25 years or so, since 1707, sure. But that’s skewed by two major outliers - Victoria (63 years) and Elizabeth II (70 years). Between the other 11 unitary monarchs the average is more like 17 years. Shorter turnovers are possible - four kings lasted 10 years or less. Not sure you’d get great odds on Charles beating that spread.