"Visiting Narnia one always finds that more time has passed there than on Earth, but there does not seem a fixed rate: between The Lion, The Witch and The Wardrobe and Prince Caspian, about a thousand years have passed in Narnia, but between that and The Voyage of the Dawn Treader only three years"
Or if you needed information in this direction, the last book was published on September 4, 1956.
Have a nice day!
I hope this is what you are looking for, but if not - comment! I will edit and update my answer accordingly. (ノ^∇^)
- Heather