Hi BobinCA,
While there is no way to specifically quantify how long any mattress will last for a specific person or predict exactly when they will decide to replace it because it is no longer suitable or comfortable for them (because this is the only real measure of durability or the useful life of a mattress that really matters) and because there are too many unknowns and variables involved that are unique to each person … if a mattress 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) and you have confirmed that it meets the minimum quality/durability specs that are suggested in the guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with higher quality and more durable materials like latex or higher density memory foam or polyfoam (in the comfort layers especially) it would likely be in the higher end of the range or even longer and the chances that you would have additional “bonus time” would be higher as well.
While you are “close to the edge” … you are a little higher than the weight range where I would normally consider a mattress that uses more than “about an inch or so” of 1.8 lb polyfoam and if I was in your shoes I would tend to look for a mattress that used a little bit higher quality and more durable materials that would be less risky in terms of durability or at the very least if a mattress contains materials that are slightly lower than the minimum guidelines relative to your weight range that I would normally suggest then it would be reasonable to reduce your expectations about the durability and useful life of the mattress and how long it may be before you may need to replace it accordingly.
Phoenix