The book series will nonchalantly say "hundreds" or "a thousand" or "thousands" of years in different places throughout, but is it ever stated exactly how much time passed between Peter, Susan, Edmund, and Lucy's first departure from Narnia and their first entrance back into it?
In this Timeline which is generally believed to be accurate as some if not all are based on actual notes by CS Lewis. We see its been almost 1300 years since the Lion, the Witch, and The Wardrobe.
Between their departure from Narnia and their return, 1288 Narnian years passed (in one Earth year):
The events of The Lion, the Witch, and the Wardrobe occured [sic] between 1000 and 1015 NY [Narnian Year], and in 1940 AD on Earth.
Due to the anomalies found between Earth time and Narnian, this story takes place in Narnian-year 2303 and Earth year 1941.