The books imply that the Pevensies were in Narnia for years (or perhaps even decades) in The Lion, the Witch, and the Wardrobe. Is there any indication of exactly how long they were gone?
The original rule of the Pevensies lasted for 14-16 years, starting in year 1000 and they returned to London in either 1014 or 1015 while hunting the White Stag.
The interactive scrollbar lists the time as 1000-1014, while accounts on the web vary between 1014 and 1015 for when they actually returned. So depending on the months, between 14 to 16 years is the time of ruling.
These come from something Lewis created himself, "Outline of Narnian History so far as it is known...", as referenced in "Inside Narnia: A Guide to Exploring the Lion, The Witch and the Wardrobe" (Page 79). This references a couple of other works, one of which is "Past Watchful Dragons" by Hooper, which reprints the timeline (Pages 41-44).
I could not find any direct publications by CS Lewis, just references to this timeline and publications featuring it.
We know, that at the time of The Horse and His Boy, Rabadash had asked for Susan's hand in marriage. We are also told that he was not the only one.
Assuming that the invitations started when she was 18, and that ten people proposed to her, at the rate of five per year, she was 20 years old at the time of THaHB. Since she was 12 at time of entering Narnia, we can assume that the Pevensies were in Narnia for at least 8 years.
The Narnia Wikia says
This took place in 1000 NT, and she and her siblings shared rule of Narnia for the next fifteen years.
So, 15 years.
This comes from here.
See also Golden Age of Narnia.