How many years were the ottomans an important threat to european/christian countries? (start from the date of that city's capture and round to the nearest hundred.)

Byzantium/Constantinople was conquered in 1451, but the empire continued to decline starting around the 18th or 19th centuries. It officially died in WWI. As for how long it was a major threat to Europe, the empire's prime was the 16th century, under Suleiman the Magnificent.