The average generation is about 25 years long, though that depends on how early in life people tend to have children. If, in your civilisation, 16 or so is the average childbearing age, you're going to have shorter generations--say nineteen years or so. But if it's generally happening later in life, like 30-40, the generational gap will expand to 30 or more years.
From those calculations, as well as the numbers you yourself have run, about 150 years seems to be the maximum--we're talking four generations here, right? I'd really say go more for 120 or so. If this is a fantasy world based on medieval times, it's likely that the childbearing age would be twenty or below.
"Wise men speak because they have something to say; fools speak because they have to say something." - Plato