Hi CentralPA,
There isn’t any way to know for certain because it will depend on many factors including your weight and weight distribution, sleeping positions and style, and where inside your range of “comfort and support” you are when you sleep on it. It could be anywhere from a few months or less (if the initial break in period makes the mattress too soft or unsupportive for you) to a few years or in some cases perhaps longer if you are closer to the “I can sleep on anything” end of the range than the “princess and the pea” end of the range. There is more about the factors that can affect durability and the useful life of a mattress relative to each person in post #4 here and the posts it links to.
Phoenix