Hi Nemix,
Unfortunately there is no way to quantify how long a topper will last for a specific person or predict exactly when you will decide to replace it because it is no longer suitable or comfortable for you (because this is the only real measure of the durability and useful life of a topper that really matters) because there are too many unknowns and variables involved that are unique to each person (including their weight, their sleeping positions, their preferences, the firmness of the topper, and the specifics of the mattress under the topper). If a mattress/topper combination is well inside a suitable comfort/support range and isn’t close to the edge of being too soft when it is new (see post #2 here) then it would probably be reasonable to expect a useful lifetime somewhere in the range of 5 - 10 years (regardless of the thickness).
Phoenix