Hi buttercupbetty,
In my conversations with quite a few manufacturers of wool toppers of different types (see post #3 here) there was a wide variance of how long they each thought they would last most people “on average”. The biggest reason that people replace them is really personal preference when the wool has compressed more than they are comfortable with or for hygienic reasons because the wool itself won’t “wear out” for many years. Wool fleece will compress and matt down more than wool batting but it has a softer surface “feel”.
The most common time estimate seemed to be in the 5 - 10 year range but they all indicated that some people use them for longer than this and they were all clear that this was very much a matter of personal preference and not because the topper had “worn out”.
Phoenix