What is the period of time Narnia took place?

The books span the entire existence of Narnia from start to finish, the first book (the Magician's Nephew) shows the very beginning of Narnia, and the year 1900 on Earth. The second takes place much later in Narnia, and around WW2 era on Earth. The last book shows the downfall of Narnia and it is unclear what period it is on Earth, although Peter, Eustace, Lucy, Jill, Polly, and Digory are still alive.